Now that I (we) know you are in MN. Most of the trails you ride here are what I would call buff singletrack. Most trails are built to IMBA standards, and each trail has their own difficulty range (some of the features have walk/ride arounds).
To start with, go check out the MORC forum (
http://www.morcmtb.org/forums/forum.php). This is the local MN MTB group that is heavily associated with IMBA. Most of the Twin Cities trails are MORC trails (except Elk River MN, which is a whole different beast in its own). If you are further up north, you have Cuyuna in Crosby (
http://cuyunalakesmtb.com/), which is awesome single track network. OR further east and north, you have COGGS in Duluth (
http://www.coggs.com/). These are pretty much the main groups in MN, but there are others (watch for GRIMBA in the Grand Rapids area as they are about to start something really big).
MORC has a used/for sale section that is pretty good, and you will find some decent deals on used stuff there. And of course hit Pinkbike and Craigslist as has been mentioned before.
Now into what I would look for with a beginner. First off, figure out who your friends are that you will typically ride with. Find out what they ride, as this will dictate a little of what you should look for. IF they are riding full rigid carbon race bikes, you are gonna want to be in the same category (otherwise they will quickly outclimb and out pace you), and vice versa if they are riding 6" travel full squish endure style bikes.
For a beginner in MN, I would suggest a steel or aluminum framed option. I would personally go hardtail with front suspension in your price range. The full suspension offerings in the $1k under range are not what I would call suspension. For MN a 80mm to 140mm fork (120mm perfect) is all you will need, unless you plan on hitting huge drops to flat (that's a whole diff story). Air or spring, most people love air forks, but I personally prefer springs when you know how to dial them in (springs can be a lot more linear in their travel vs air). Springs will take some work and $$ to dial in though, where as air forks only require a pump and patience.
As for tire size, you are tall enough to get away with wagon wheels (29"). Bike size will most likely land you in a Large an potentially a Medium frame size. Its a pick your poison on wheel size, but for MN 29" is the most common in XC and trail style bikes. Wheel size(s) are like asking if you should run flats or clipless pedals (everybody has an opinion). FYI, on pedals; I run flats on all my bikes (with 5.10 shoes); but that is my preference/opinion.
Some bikes I would look at are as follows:
Kona Honzo - (closeouts will be closer to your price range)
http://konaworld.com/honzo.cfm This is one of the funnest 29'rs I've ridden. Really playful and can take some abuse.
Kona Explosif - There is a medium at Behind Bars (I think) still a little high in your price range tho.
http://konaworld.com/explosif.cfm This bike is 27.5, and is an absolute blast to ride for a hardtail.
Kona Taro - Aluminum version of the Honzo. Haven't ridden one as I prefer steel or Ti for hard tails, but should be a good and fun bike. Holdvers wil easily get you in your price range (once again I know Chuck at Behind Bars has a few of these as hold overs).
http://konaworld.com/taro.cfm
Trek Staches look like fun bikes from geometry, but personally haven't ridden one (since they first launched, but their new 29+ looks like the bike to have in MN). Might be able to find a hold over to meet your price range.
http://www.trekbikes.com/us/en/bikes/mountain/trail/stache/
Trek X Calibers are XC hardtails that fit into your price range also.
http://www.trekbikes.com/us/en/bikes/mountain/cross_country/x_caliber/
Salsa El Mariachi is another hard tail you might want to look at (personally I would take this over a Trek X Caliber).
http://salsacycles.com/bikes/el_mariachi
Hopefully that gives you a start/idea. As stated earlier, don't be afraid of looking into used stuff, as you can get some good deals on used stuff (but you do have to be careful). Just buy what you like, start hitting up bike shops and ride a bunch of different ones and sizes and see what you like, Also if you start watching the MORC site, demo days should be starting soon. This is where you will b able to try out different bikes at different trails.
