Hi Leonidas,
Don't listen to your friends! You don't NEED to do anything but go take photos, learn to use what you have and surprise your friends with what's possible!
The EF-S 55-250mm IS STM is actually quite good. There would be little to be gained getting one of the 70-300s instead. In fact, the 55-250mm is smaller, lighter and closer focusing than either of the more affordable 70-300mm currently offered. And it's got just as good image quality. The only differences are that the 300mm offer just a little more focal length (all of 50mm) and a bit faster autofocus (USM vs STM). The 70-300s are also usable on full frame cameras, while the 55-250mm can only be used on crop sensor cameras like yours. But, primarily, based upon what you say you like to shoot, this will probably be one of your least used lenses! Keep the 55-250mm and use it! If you don't already have it, get the ET-63 lens hood for it (Canon's costs about $24 US, but there are cheaper clones such as Vello that will probably do the job just as well).
Your other three lenses essentially duplicate each other. Pick among them the one you like best and sell or trade in the other two. You say you're already selling the 18-55mm, so let's consider the other two "walk around zooms" instead. There are three different EF-S 18-135mm IS... the original with micro motor focus drive, an upgraded version with STM focus drive and the latest with Canon's new "Nano" USM focus drive. The last two are pretty much identical optically and show some image quality improvement over the original. They both also have quieter, faster autofocus... the newest version with the Nano USM being the fastest of the three.
Something interesting to me about the 18-135mm is that it's full frame equivalent would be roughly 29mm to 218mm. Back in the days of film (which is "full frame"), that range of focal lengths was just about all that most people ever owned and used in their lifetimes. Many cameras came with a "normal" lens around 50mm. Folks then often added a telephoto zoom such as an 80-200 or 70-210. Finally, they often got a wide angle to complete their "kit", most often a 28mm. So, in a sense, in that single lens you've got all the focal lengths that many film shooters ever owned and used. Even better, since it's a zoom you have all the focal lengths in between, covering all the "gaps" in the film shooters' kits!
If yours is the STM or Nano USM version of that lens, it probably has better image quality than the much older EF-S 17-85mm IS USM. If yours is the first version of the 18-135mm (not marked STM or USM), it's probably about equal for image quality, but slower autofocusing than the 17-85mm with it's USM focus drive. Best thing you could do with those three lenses is try them all... shoot some tests with them. Pick a highly detailed subject like a weathered wooden fence or brick wall and take shots with each lens wide open (largest possible aperture) at their most wide angle, one or two "middle" focal lengths and their most telephoto settings. Then compare the images on your computer. Pay particular attention to the corners of images, as that's where lenses are usually at their "worst", and see if one or another stands out as a "keeper". If so, great.... you'll know what to do. But if you don't see much difference, you may need to consider other differences such as size, weight, focus speed, convenience of the focal length range, the effectiveness of stabilization, filter size (same as other lenses, so they can share), etc. It might even just come down to one or the other "feeling" more right to you. Whichever you end up keeping, be sure to get the matching lens hood for it and use it.
Note: For lenses you don't have and can't test yourself, I recommend Bryan Carnathan's the-digital-picture.com website. There you can compare the image quality and many other factors of almost any lens made to fit Canon (as well as a growing number of other brands of lenses). His reviews are good and thorough, but the magnified test target shot comparisons and other tools available there make it very valuable. (If you use that site, just be sure to compare lenses with as similar cameras as possible. Most he shoots with two or more cameras.... You might not find any done with an SL1/100D, but a lot have been done with 60D or original 7D, both of which use essentially the same sensor as your camera. It's not always possible to use exactly the same camera... just don't accidentally compare lenses using a 50MP full frame camera like 5DS-R with it's cancelled low pass filter versus another lens on, say, an 18MP APS-C format 60D.)
Based on you liking to shoot landscapes, in particular, I'd recommend you consider adding a Canon EF-S 10-18mm IS STM or EF-S 10-22mm USM. I think you'll find a wider lens such as these most useful for landscape photography, and possibly for the night sky too. The EF-S 10-18mm is a relatively "kit quality" lens, has surprisingly good image quality for a bargain among ultrawides at under $300 US. It's also one of only two lenses of this type with image stabilization (Tamron recently introduced another). The 10-22mm is an older model, but a bit bigger and better built... and has been one of the best lenses of this type made by anyone for a long time now. But it's about twice as expensive as the 10-18mm. It also uses larger filters (10-22mm: 77mm, 10-18mm: 67mm).
A couple alternatives to either of those Canon UWAs ("ultrawides") are the Tokina AT-X 11-20mm f/2.8 DX or the earlier model 11-18mm f/2.8 DX that it superseded. These two are particularly popular for night photography, because their large f/2.8 aperture helps brighten up your viewfinder. No other UWA offers that large an aperture. The Tokina also are very sharp lenses. The older one actually might be the sharpest, but is very susceptible to flare issues (in contrast, the Canon 10-22mm is one of the most flare resistant ultrawides). This can be a consideration with UWA lenses, because of their broad angle of view you will more frequently see the sun or other strong light sources within or near the edges of your images. The newer version does a lot to reduce flare.
All the above lenses are zooms and most of them have relatively small maximum apertures. The Tokina I mentioned with their f/2.8 offer the largest, so theoretically would be best for low light shooting. The reason folks are recommending the EF 50mm f/1.8 STM lens to you is because of it's larger aperture (more than twice as bright as f/2.8.... and more than 4X or even 8X brighter than some of the other lenses). It's also compact, lightweight and CHEAP at about $125 US. On an APS-C camera like yours, it acts as a short telephoto and is a wonderful portrait lens for the money. It doesn't even really need a lens hood, because it's front element is pretty well recessed and shaded by the lens barrel itself. HOWEVER... you didn't express an interest in portraiture. And, there are alternatives. Canon themselves offers an ultra compact 40mm "pancake" lens that used on the diminutive SL1 would make for a very discrete "street photography" camera, maybe even "pocketable" (provided you have a fairly large pocket). Personally when I'm using my APS-C cameras, I carry a Tamron SP 60mm f/2 "macro/portrait" lens. It's more expensive than either of the Canon lenses (and is rather slow focusing), but in addition to being a nice portrait lens and relatively compact, it also gives me full 1:1 macro capabilities. That takes the place of
three lenses I used to carry (50mm f/1.4, 85mm f/1.8 and 100mm macro). But what works for me (or for your friends), doesn't necessarily work for you.
There is nothing at all wrong with your SL1 camera. It's still the smallest and lightest DSLR that anyone has made. It's fairly simple to use and should be quite good to learn with. However, be aware that the SL2/200D got some nice upgrade attention (unlike Canon's more entry level T7/2000D versus T6/1300D, which appears will be identical except for an upgrade from 18MP to 24MP). The 18MP sensor in your camera first was used in the original 7D (I shot with a pair of those for over five years). It was fine, but there have been some advancements in image sensors and Canon is now using a 24MP in all their APS-C cameras... both DSLR and mirrorless (M-series). The new sensor is not only 33% higher resolution, it's also got close to a stop wider dynamic range and is able to be used to higher ISOs (compared to my older 7Ds, I'd estimate at least one stop higher usable ISO... maybe two). That's a noteworthy increase, especially when considering that resolution was significantly increased, too. The Canon 80D was the first model to use the new 24MP sensor and it certainly got everyone's attention. As of now, it's been "trickled down" to all the other APS-C Canon models. Only one doesn't use it (the 7D Mark II has it's own unique 20MP sensor, somewhat derived from what was used in the earlier 70D, but with some serious tweaks).
There are lots of reviews of SL2 and comparisons with SL1 online (such as
https://www.imaging-resource.com/cameras/canon/sl1/vs/canon/sl2/), but I wouldn't be too quick to go looking at those. That's something for the future. No need to "get a better camera" right now. The SL1 is quire capable and lightyears ahead of DSLRs anyone was producing only 10 or 12 years ago. I would note that it uses a pretty small battery with fairly limited shots per charge, so if you don't already have one you might want a spare to keep handy, just in case (the SL2 gets nearly double the shots per charge on average).
At this point, I'd recommend you put more of your effort into learning photography, than adding, trading or upgrading gear. A book like Bryan Peterson's "Understanding Exposure" might be much better use of your money, than spending it on gear. There are a couple things, though. First, think about getting a good tripod... you'll need it for those night shots and might want to use it a lot for landscapes, too. DON'T BUY TOO CHEAP a tripod... They're junk, don't work well and you end up replacing them every year when they break. Over the long run, a better quality tripod probably ends up saving you money. After all, a good wont that holds up over time might well serve you throughout your lifetime... it's not like tripods are seeing a lot of innovative new features we "just gotta have"! I mentioned spare battery and lens hoods for your lenses. I'd also recommend one type of filter.... a quality Circular Polarizer to fit the lens(es) you use most for landscape photography. C-Pol remain one of the most useful filters in the age of digital photography. They can serve many purposes, but in scenic shots they deepen the blue of the sky, enhance clouds, reduce reflections off water and foliage, often increasing color saturation, and more. Almost every other type of filter is unnecessary with digital photography (the camera itself or post-processing software can emulate most of them... even do a better job that some filters ever could). Another type of filter that's popular for scapes is a Neutral Density... used to allow longer shutter speeds in brighter light conditions... but that's a somewhat specialized technique, where C-Pol's uses are a lot more generalized.
Finally, you probably got a user manual with your camera... if not you can download a free PDF from the Canon website. But you also might want to look for one of the guide books specifically for the SL1. Those can be helpful getting up to speed using the camera to its fullest. I always make a point of buying a guide book, any time I get a new model. It just helps shorten the learning curve, regardless of the photographer's level of experience.
Enjoy your camera and whatever lenses you settle upon.
And.... ignore your friends and their opinions! (You know what they say about opinions, don't you? I won't repeat it in polite company.)
Instead, "Wow!" your friends with your photos! That'll be sweet revenge. Next they'll be running out to buy the same camera and lenses you're using. After all, we know photography is like great meals, it's the "good pots and pans and an expensive stove" that make for a great meal. The chef doesn't have anything to do with it!
EDIT:
Beware good intentions and advice on forums! The NIKON mount lenses suggested above will not fit or work on your CANON SL1. And the 12mm Rokinon recommend in another response is for mirrorless cameras (such as Sony A6000 or Canon M-series). Yeah, it's available in Canon EF-
M mount,
which also won't fit or work on your SL1 (which uses EF & EF-S lenses). There is another Rokinon 12mm avail.in EF mount that will fit and would work, but it's a fisheye lens which will render heavy curvature distortion in most anything photographed.
Finally, "don't by EF-S, only buy EF lenses because some day you may get a full frame camera that requires them" is terrible advice. For one, you may NEVER need or want a full frame camera. APS-C have come a long way. Your SL1 can shoot circles around a full frame 5D from 2003. Likely APS-C will continue to improve. And not everyone NEEDS full frame. In fact it means less lens selection, bigger/heavier/pricier gear, and, unless you make really big prints, it's hard to tell any difference. All too often, pixel peepers looking at their images on their computers at extreme magnification (i.e. 100%) are the only people who actually ever see all the "goodness of their full frame camera". By the time the image is resized for display online or to make an 8x10, there's little difference to be seen. Besides, there are some great EF-S/crop only lenses you'd be cheating yourself out of.
Hi Leonidas, br br Don't listen to your friends! ... (
