Exactly my advice , and with the newer 27.5 , if you have the room you can run the 6Fattie's and have UNREAL climbing grip .
I saw some of these at a bike shop. They seamed better priced than similar moe popular bikes http://khsbicycles.com/
This - I just got a full suspension Specialized Camber ($2200 bike) for $750 on craigslist. It was 2 years old, breaks needed bleeding, and needed some tuning, but it was a great deal. I wasn't sure I would "love" the MTB as much as the road and track, so I didn't want to invest big $$ in a bike. Lots of great used bikes available.
I have a khs sixfifty that I've really liked. It has a 1x10 setup that I love due to a ton of problems on my old bike's front derailleur. Before this I had a 26" hardtail khs with cable disk brakes. This bike is so much better for just about every type of riding besides.
I think what I'm going to do in the short term is demo a bunch of bikes. That'll give me a better idea of what each bike has on offer.
Coming from a 26 full suspension, I really like my current 29er hard tail. Even locked out, the previous shock would make climbs less than ideal. The HT climbs great and doesn't give up much on the downhill stuff. I demo'd a couple 27.5's and they felt nice, but the bigger 29's just feel good almost everywhere.
Buy off criagslist. Always great deals to be had from people who buy top dollar bikes that just sit in the garage.
Fwiw, I'm on a carbon HT 29r (100mm front). Been on 26" forever until 3yrs ago. From 140mm both ends, to 80mm (Fox "Brain" fork. pretty harsh) HT 26" XC race-only bike. I'm 58 with bad knees (no ACL in one of them) from MX in the 70s. The HTs make no difference regarding the knees. At least for me. More-so ones back ,when hammering the faster rougher choppier stuff. Also I've crushed 7 back verts and a couple 2x broken necks a few yrs ago..its still absolutely no big deal.
As far as budget bikes go, I don't think this can be beat. http://www.raleighusa.com/tokul-3 If you're being realistic about how often you'll really be riding it, it makes even more sense.
I am also really considering fat tire bikes. They're fun, and because the tires are the suspension, that's 0 maintenance or cost to maintain.