I have a bering optics hogster R25 and an AGM TC 35-384. I use the clip on exclusively. The advantage of the dedicated thermal is that it weighs less overall. With the clip on you not only have the weight of your scope but also the weight of the clip on and the clip on weight is forward on the rifle which may not be ideal for your situation. I hunt blinds so its no big deal for me. I also like that when I start hunting during daylight hours and transition to night, I just clip the unit on and I don't have to worry about removing my daytime scope with quick detach mounts and putting on the thermal. People will tell you that you can hunt daylight with a thermal which you can but it sucks in my opinion. The other advantage of the clip on is that you zoom using your scope. You can zoom up to 6x. With the dedicated thermal, every time you zoom you cut your resolution in half. If you go dedicated, I recommend getting a higher base magnification so you have to zoom as little as possible. This is why I use my clip on exclusively. My hogster has a base magnification of 1.4x. At 1.4x, the picture is awesome but if you zoom it up twice, you are looking at blobs with no definition. I leave it on 1.4x and never zoom it. If you are going to hunt coyotes or hogs at 150 yds or more, get a 640 base resolution with the highest base power you can afford. Unfortunately, finding a budget 640 resolution with a 4x base magnification is going to be hard to find on a low budget.
I doubt you will find a low budget clip on used. AGM has the TC19-256. I have not looked through one so I can't make a comment. It would probably work ok for distances under 100 yds. I would stay away from the ATN thor lt 160's. I have never looked through one but 160 is the base resolution. AGM has a couple of 256 resolution scopes for close to the same price. There is also the hogster stimulus. I also don't like built in batteries. If your battery goes bad then you have no thermal until it gets back from being repaired. Both AGM and Hogster use cr123a batteries, both rechargeable and non-rechargeable so if something happens during your hunt, you can always run down to walmart and get more batteries if the need arises. You will probably get sticker shock and pass out but at least you can get them if needed.
As for buying one used, I can't give any advice other than to buy a name brand with some warranty left on it. I can say that AGM has an awesome customer support department. They are very helpful and quick to respond. I am kind of in the same boat in a way in that I want to sell my hogster but not sure of the best way to do it. People are reluctant to buy used thermals and I don't blame them.