It all comes down to what type of trails you want to be riding, and what type of rider you are. On a 29er, a hardtail is fine, but I wouldn't limit yourself to just a 29er. Sure they are new and all, but a full suspension 26 inch wheel bike can handle way rougher terrain than a hardtail 29er can.
As far as brakes go, you want hydraulic. Mechanical disc brake are nothing more than a pretty looking rim brake. Hydraulic disc brakes have way more stopping power than mechanical ones do. You can stop easily with just one finger, necessary if you're doing technical riding.
Look on pinkbike as people have already said, it is the ns of biking.
With a budget of $800, you can get a damn good used bike.
Things to look for:
-Components: Pretty much what makes the bike work. SRAM and Shimano both make different level of components. XT is what you want for shimano, and x7 is what you want for SRAM.
-Brakes: Hydraulic disc, only way to go. If the bike you are buying is a nice mountain bike, and it was made in the past 6 years, it will have hydraulic disc brakes.
-Wheels: Make sure they are true, otherwise you'll need to buy a new wheelset down the road.
-Pedals: Personal preference. If the bike has clip in pedals, try them. They are awesome
-Frame: Shouldn't have any cracks. Its not like skis where you can fix a core shot. A cracked frame is bad news
-Fork: There should be no leaking, and all of the levers and knobs should still preform their intended function. Fox, rockshox, xfusion, marzoochi, all good brands.
Like I said, don't limit yourself to 29ers. You can find $2-3k bikes, a few years old, in your price range. I found a 2k bike from 2008, got it for $450. Just look and see what components it has. If you find a full suspension with xt components and hydraulic brakes in your range, jump on it.