At your size a 29er is the best option in my worthless 2cent opinion. I sold both of my 26" MTB after taking a "chance" at buying a used SS 29er. I have three 29ers now(geared full squishy, SS, and fixed).

The Monocog is a great deal, and a great way to get into SS 29ers on the cheap. It is a porker, but that can be fixed depending on your pocket book. The wheels are a good place to start to loose some weight. If you want to build your own also take a look at the Q-Ball frame from Scott Quiring (http://www.qballbikes.com/index.html). I own a Karate Monkey and love it, but if I were to build another one right now I would buy a Q-Ball in a heartbeat. The horizontal drops on the Monkey are a pain to deal with when using disc brakes. The Q-ball has a great sliding disc mount. Plus, it has a beer bottle opener built into the frame!!

A Q-ball/Monkey build is going to be way more expensive than the Monocog, but you will be able to pick and choose your own components.

I am 6'6" 280lbs btw and my fat boy proof Karate Monkey comes in at 28lbs with rigid carbon fork and one of the fattest tires/rim combo (Weirwolfs/Kris Holms).