More than anything else, make sure you get sized or at least get the RIGHT size. The guys at Handlebars in Kenmore are awesome, and they custom size everyone for new bikes. They sized me for a roadbike and the difference was unbelievable. I would also recommend Campus Wheel Works on elmwood for anything Jamis or Redline, see Ethan and tell him Tom with the Monocog sent you - he’s a great guy.
29" are awesome, most of the guys winning the races this year were riding 29’ers contrary to what most of the haters will say… I personally ride a redline monocog 29er with Avid BB7 Discs.
Singlespeed mountain biking is not for everyone, but it brings back a thrill I have not had not on a bike since I was a kid. It is liberating not having to deal with gears or thinking about gears, almost completely maintenance free, and will get you in shape in no time. My bike complete with brakes was in the $600 range, which is dirt cheap for a bike of that quality. The only thing I might do this year is add a fork, unfortunately the 29" fox is more than the cost of the bike, but should be worth it. Riding rigid is fun, but it beats you up.
FYI, I am 6’3" 240lb and I beat the shit out of the bike, and the only money I spent on maintenance was a new $30 bottom bracket (that I think was installed incorrectly from the factory.) This brings up another good point - don’t really worry about weight if you are trying to get in to it on the cheap, it really doesn’t matter from a fun perspective, or a workout perspective. If you want to get into racing someday - then buy a race bike and spend the big money it takes to get rid of the weight.
One thing noone mentioned is that you will need a few hundred dollars worth of stuff on top of the actual price of the bike, so budget accordingly. (Shoes, Gloves, Pedals, Camelback, Helmet, Tools, Tool Sack, Air Pump, bike rack, Lube, etc) I probably have as much into this stuff as I do into the bike itself.