That’s a pretty open question, it’s like asking us what book do you want to read? Without any other info about your preferences….
So the bike you should be looking at, is the type you want…
No point in looking at a pure XC speed machines, if you want something trailsy. Decide what you want and what you really can get for your money (by looking at ad’s), and then put up a WTD ad asking for that is a good start… Where should you look? Here (WTD ad/for sale sections), ebay, gumtree, pinkbike. No reason not to look at 29ers, though the newer more progressive (traily) types won’t be as easy to get with your budget. Though that’s fine if you want a fast XC 29er.
I’d suggest sticking with 26er’s, unless you quite tall, as 29ers tend to suit taller 6’+ ppl more often, though everyone’s different
Gunna give us any more into to work with?