I'm a yamaha guy so on the smaller side I would recommend a 250 Timberwolf or a 350 Bruin.
in the middle of the pack a 450 Kodiak is a good working machine and they usually come with locking diffs and they have a low range.
I have a 550 Grizzly myself which I really like but thats probably not in your price range as they are somewhat newer machines and yes used ATV prices are pretty ridiculous right now.
as far as the durability of the Kodiak that I mentioned I co-own one with my parents that is pushing 6500 miles and its never needed any engine work, the only thing major is I had to put a new wet clutch in it last year because it was worn out, but we use it for hauling wood etc so Its definitely a working machine but the engine is very durable and its a very capable machine.
If you can find one that needs minor work its a pretty good deal, I was able to fine another kodiak for less then 2 grand that needed some minor repairs and it came with a plow.