I know where you are at. 100 lbs in three years??? I am 6'8" and 360lbs in my late 50s and it is getting tough to keep the beef and lose the blubber.

You might want to look at the blood type diet, it lays out different foods that can be good for your blood type, or real bad for you that you should avoid.

You most deff want a taller bike, but plan to become a master at truing your wheels. I was going through stock wheels way too fast but have found a wheel builder that can do custom for big guys:
http://www.prowheelbuilder.com/cw/ Go to MTB XC section and select the build...

The VELOCITY CHUCKKER MSW 26IN BLACK FRONT RIM is almost bomb proof, and will take the weight. Go with the 36 spokes, and see if you can swing at least getting the rear with SHIMANO XT M770 REAR $159.60 .... Still under $300.00 including the $125 for the bike, and you are all set to go and safe. This rim will out last that bike and will be with you on your next.

My biggest problem with stock wheels is popping the spokes out trying to climb hills in the wrong gear, too much power + wrong gear = ZPROINGGG, and once they pop they are never the same.

Just make sure that you know what size the spacing’s for your axels are. Mt bikes are usually 100mm front, and 135mm rear. Road bikes 100mm front and 130mm rear, and the older bikes with the 27 inch wheels are 126mm rear.

Go for at least 36 spokes for strength 40 spokes better, a Mountain bike that has a 26" wheel will take more weight, shorter spokes, beefy rim. This Specialized RockHopper Mountain Bike - $125 is real nice for you to get started on if it does not need too much. If it is ready to ride jump on it!

You are doing the right thing getting out there, you will make it, N E V E R give up.
DaPoppa