Regarding dust... all lenses have some inside. Shine a flashlight through any of your lenses and you will see some specks inside. Lenses aren't assembled in "clean rooms". They don't need to be because a little dust generally has no effect on images.

However, it depends upon what "light dust inside" really means. What one person calls "a little" another might consider "a lot". For the large part, MPB seems pretty good and their description "no effect on images" is probably correct. Plus I think they have a reasonable return policy if you are dissatisfied for any reason (but check this before buying).

As to the lens itself. The Sigma 120-400mm was made in at least a couple different versions. The later one was improved with image stabilization added. Sigma calls it "OS" or "optical stabilization". The earlier version lacked OS.

The 120-400mm (as well as the Sigma 50-500mm, 150-500mm) was among the very first "super telephoto" zooms offered at reasonably affordable prices. Upwards of 300mm previously there had only been very pricey 400mm, 500mm, etc. primes. Few zooms and much of it not very affordable stuff!

The 120-400mm is NOT the sharpest pencil in the box. It's a little better than the Sigma 150-500mm that was being offered at the same time. The Sigma 50-500mm from that time frame was actually the sharpest of the three, but also is quite large and heavy.

The 120-400mm would very likely be better than your 70-300mm used in the camera's "crop" mode. When you do that, you're basically just throwing away most of the camera sensor's capabilities. In 1.4X mode you lose close to 60% of the camera's megapixels! 2X would be even greater. In fact, there really is no good reason to use the camera's crop mode. There's no "magic" happening inside the camera. You can do the same crop in post-processing and get the exact same results. In fact, it would likely be better to do the crop in post because that allows you to adjust the size and position of the crop, if needed (as it often is... at least for me).

The 120-400mm would probably be better than your 70-300mm fitted with an actual 1.4X teleconverter (even though this DOESN'T throw away gobs of what your camera sensor captures, the way the in camera crop does). Usually when optics are combined there is some loss. It varies wildly depending upon the quality of the lens, the quality of the teleconverter and how well they work together.

But be aware it's an older lens design and there are newer lenses that are better. For example, at the link below I've compared image quality test shots from the Sigma 120-400mm OS (late) with the current Sigma 150-600mm OS "Contemporary", which is one of the most affordable that can now be bought brand new (and is also widely available used). There also is a more robust, better sealed, but bigger, heavier, more expensive, more pro-oriented Sigma 150-600mm OS "Sport". And there is a Tamron 150-600mm VC (similar optical stabilization) "G2" (2nd generation, optically improved over the first version). Notice that at the link below the test shots were taken with a full frame Canon 1Ds Mark III, which is about 21MP. Your Sony a77II camera is an APS-C model, with a smaller sensor, so only the center and midframe of those test images will apply. The far corners will naturally be cropped away using any of these lenses on your camera, due to the APS-C sensor. With your camera's 24MP resolution on APS-C, you will likely see more image quality degradation than is shown by a 21MP full frame camera. But that's what was used for the test shots being compared and is all we have to work with here:

https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=965&Camera=453&Sample=0&FLI=5&API=1&LensComp=990&CameraComp=453&SampleComp=0&FLIComp=5&APIComp=0

Note that the more modern 150-600mm is sharper at 400mm than the older 120-400mm. If you wish, you can change the parameters at that website to compare other focal lengths and/or to stop the lenses down to see how that improves image quality. You also can select other lenses to compare (but I recommend trying to compare on the same or at least fairly similar cameras... doesn't matter much what brand, just that the cameras are reasonably similar).

Your camera is what Sony calls "a-mount" (used on their translucent mirror DSLRs). They have discontinued that now and although older Minolta and Konica-Minolta lenses can be used, during the age of digital photography there has been more limited lens selection available for the a-mount, versus Sony's own "e-mount" that they use with their mirrorless cameras and continue today. Because of this, you might not be able to find some lenses in the a-mount you need. EDIT: For example, I just did a search and don't find either of the Sigma 150-600mm available in a-mount. Also, the current Tamron 150-600mm "G2" doesn't appear to be offered to fit your camera. However, the original Tamron 150-600mm DOES appear to be available in a-mount and MPB has a used one they describe to be in "good" condition, with all its accessories such as hood, tripod ring and caps, and moderately affordable at a bit over $600: https://www.mpb.com/en-us/used-equipment/used-photo-and-video/used-lenses/used-sony-lenses/-tamron-sp-150-600mm-f-5-6-3-di-usd-sony-a-fit/sku-1422120/?utm_source=google&utm_medium=surfaces&utm_campaign=shopping%20feed&utm_content=free%20google%20shopping%20clicks&gclid=Cj0KCQjwz7uRBhDRARIsAFqjuln_TRfF3NMdxyz2edcb_XrwKrqzlnBq8VvTf5l9wm0jwX5kPhfOvAEaAjAZEALw_wcB&gclsrc=aw.ds

I've set up another image quality comparison, this time between the original Tamron 150-600mm and the even older Sigma 120-400mm, both at 400mm: https://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=965&Camera=453&Sample=0&FLI=5&API=1&LensComp=929&CameraComp=453&SampleComp=0&FLIComp=3&APIComp=0

Once again, both are tested on full frame, 21MP Canon 1DsIII.

You might find some other lenses to consider. I've just used the 150-600mm's as examples because they are newer and, in addition to better image quality, for just a little more money would give you a whole lot more "reach" than that older 120-400mm. For wildlife and especially birds, you may not find 400mm all that much improvement over your 70-300mm lens. BTW, at The-Digital-Picture website you also can compare other factors, such as lens flare resistance, optical vignetting, distortion and general specifications. Looking at the latter I was a little surprised to see that the Sigma 120-400mm and Tamron 150-600mm weight isn't all that different even though the five year newer Tamron is somewhat larger. That Sigma is slightly under 4 lb., while the Tamron is close to 4.5 lb.

https://www.the-digital-picture.com/Reviews/Lens-Specifications.aspx?Lens=965&LensComp=929

Good luck in your search for a more powerful telephoto! Any of these options will be an improvement over the heavy crop technique you've been using!

I shoot with a Sony 77a Markii. I have been searching for a bigger lense as I do a lot of wildlife and waterfowl. I saw a member in the forum who recommended mpb to somebody else about equipment.

I found a used Sigma 120 - 400, a mount ( very hard to find as my understanding is Sony is going to start doing away with a mount) on MPB. It is in good condition and is described as having light dust in the lense which will not impact quality of photos. 349$
My question is, does light dust impact the quality of the photo??

I have never bought used lenses so I am pretty much watching what folks here say. I appreciate all your constructive input. Thanks so much.
Oh I am using a Sony kit lense that came with my 350a back in mid 2000's, so an upgrade is obviously needed. It is a 70 - 300, but my camera has button to increase focal length 1.4x or 2.0x. I use this feature frequently and do get decent results, however, I realize a lense is needed...again thanks for your input.