All good suggestions about a hardtail... I prefer 26 but frankly since the fad is fading there are alot of nice 29er hardtails out there right now for sale (it would not be my preference to buy 29 but alot of folks do). A friend purchased what was new retail around $4800 (2012 Spec. Stumpjumper carbon HT with X9 componeets and Stans wheels only ridden by a little old man on Sundays) for $2000 off of CL.

I would do your own research first to determine what bike suits your riding style best. Trails here in CA are steep, rocky, sandy and rutty - in other parts of the world they made be smooth and flowy. Just know like with everything, there is no such thing as a "jack of all trades" mountain bike. There are as many styles as there are types of riders... cross-country, all-mountain, down-hill, freeride, hard tail, full suspension, rigid, 26, 29, 650B, aluminum, carbon, steel, ti... I can go on. For a newbie - a hard tail is generally best as it forces you to learn skills first - it also provides a good price point and its generally but not always cheaper than full suspension. I bought my Juliana for $2000 and then upgraded. The HT my friend brought retailed for much more than my full-suspenion cost new.

Best suggestion, find a buddy who mountain bikes and ask to borrow a bike. Continue to do research and then buy the best bike you can for the money you have to spend. Study components and what is good, not good, a deal etc. Buying a bike is as complicated, maybe more, than purchasing a car - just have fun with the search...