42nd Street Photo has been around for decades and has always had a "dicey" reputation. B&H, Adorama, Cameta has always been great to deal with and don't play some of the standard tricks on "lower price" gear.
By way of example: The manufacturer supplies a battery and a charger. Some of the shady vendors pull this from the package and then sell you the battery separately and the charger separately. Suddenly your savings is going away. Why bother when there are too many GREAT vendors, see above, out there.
This is WHY paid photographers and videographers will always be in demand. Uninvolved parties are always the best for such emotional occasions. People don't want to see the photos and videos RIGHT after the event. I did a memorial for 39 year old school teacher and made sure all of the media was GOLD media so that when her 11 year old daughter has a child, she will get to know her grandmother through the media, which I would want to work in 30 years.
This is also why I network. Get a buddy to cover for you and work out a future trade.
Get a REFURBISHED D600 if you want one. The EARLY production models had this issue. The later ones seem to be fine. If the D600 has been REFURBISHED, Nikon, probably at the repair center, has taken a second look at that particular body, and address the issue, if it had occurred on that particular body. I would MUCH prefer a refurbished D600 to a new D600 for this very reason. The D610s have all been fine. No reports, that I can find of a problem with them.
Your old lenses WILL work on a D7100. I chose a D90 to support my 80-200 which will NOT auto focus on a D5100.
Also keep in mind that telescopes use clock drive to synchronize the movement with the camera. The moon is blurry because it was MOVING the entire 20 seconds. Anything beyond 2 seconds will give noticeable motion blur in astrophotography.
Go out to
www.bn.com and purchase the Magic Lantern Canon 60D Guide USED for $1.99. Older camera, older book, MUCH cheaper price. I buy LOTS of used books this way, saved a boatload of money and NEVER got a bad copy of a book.
If it works on a D5100, it will work just fine on a D7100. If you are looking at a new body, seriously look at the D7000 before you drop the cash on a D7100. TINY performance difference with a HUGH price difference. Just a thought.
I shoot with a D90 and swapped my 18-105 for an 18-55 VR and some cash. You can get an 18-55 VR refurbed for $99 and an 55-200 VR for $129. The two lenses will cost less than a single longer lens. The 18-140 has gotten some very good reviews. I traded off my 18-105 because I kept reading that it was rather soft at the short end. It is also a very BIG clunky lens in my book and you might be UNPLEASANTLY surprised how heavy it is, especially compared to your 18-55.
I make LOTS of money off of folks like you all the time.
First of all, go buy an OLD, CHEAP camera of the type you are looking for. Practice, Practice, Practice.
Second, the a right angle prism for your telescope. I have done astronomy myself.
ONCE you have gotten good with the basics and decided that you LIKE do it, THEN invest in the rest of your gear.
For this type of work, I would suggest a Canon 5D MARK 1. Full frame used for about $500. Lashed onto a telescope, a lot of things won't matter. I figure that you have a clock drive for time exposures any way.
How I many money off of people like you is that they spend BOATLOADS of money on brand new stuff, decide that they don't like it or are not willing to put in the time to learn how to use it well and so they sell off this barely used gear for half or less. I pick it up for cheap and resell it. I am SURE you see this with telescopes all the time too.
A fast lens in key. By fast, I mean, low aperture number.
Say a 50mm 1.8 or better yet 1.4.
If you have a Canon they even make a reasonably priced 1.0.
You will want your shutter speed around 125 of a second.
Crank up the ISO to the max WITHOUT exceeding the fastest NATIVE speed of the sensor. My D90 will go to ISO3200 native but I can go up to more notches but I can end up with noise.
I would definitely go with the NIKON 80-200. DON'T get a push pull version. Get the twist version. Expect to pay between 600-700 for a used one. B&H will sell you a brand new one for $1100. Make sure that your camera will auto focus with it. You have to have a focus motor in your camera to auto focus this lens. I shoot with a D90 and life is beautiful. If you have a D3100 or D5100, not internal focus motor will mean NO auto focus.
Nikon 80 - 200 used $600 - $700 used $1100 new.
Nikon 70 - 200 VR I used $1400
Nikon 70 - 200 VR II used $1800ish New $2200ish without buying a camera body.
All three are GREAT lenses. Tamron not so much.
In the MIDDLE 1980s, I actually SOLD memory at $1,000 per 1 Megabyte stick and you needed eight of them to make the computer work because they were 1 MB X 1 sticks!
One of me! I had my first programming class in high school in 1976 and my first computer was a TRS-80 Model I Level II in 1981. The Hard Drive I sold in a computer store was a 5 MEGAbyte hard drive for $5,000! It was an HP drive. At that rate a 2 TB drive would cost 200 BILLION dollars! Yes, things have come a long way baby.
I would love to attend your class. Windows 8 is a disaster. 8.1 not as big a disaster. I run Windows 7 Pro 64 bit with XP mode. VERY Happy! See NO reason to upgrade.
As a fossil of the technology era I am 54!