Suggestion on buying a single speed 29er mountain bike
SE Stout is a single speed mountain bike that runs relatively cheap.  

If you're confident in your ability to put it together properly, you can find them at bikesdirect for around 350. 
http://www.bikesdirect.com/products/se/stout.htm

It's got tabs for disc brakes so you could upgrade in the future, but comes with V-Brakes stock.  I have a few friends who have these bikes and really like them.