In that price range you're not going to find anything full suspension. And if you go used you're just going to waste your money (you don't want somebody elses used rear shock).
I'd recommend a hard tail to start out with. If you're down hilling with some cross country you don't need a FS bike. They're nice, but not really necessary, you're legs do most of the pumping and absorbing as it is. Unless you're legit DH'ing, FS is just a luxury.
If you're looking for a good freeride bike with a quality component package I'd recommend this. For the money you're getting a quality package. Plus the six-fifty wheel size is better than the 26 for speed but more manueverable than a 29er.
SixFifty: Jamis Nemesis 19"