I found this forum early last year while studying up on getting into riding. It was very helpful with lots of info for a new heavy rider. The weight loss: before and after pic thread was particularly motivating.
After lots of research on what kind of bike I needed I ended up finding a '94 Mongoose Switchback MTB on Craigslist for $100. Mongoose seems to have a bad reputation as a cheap big box store bike but I think mine was made back when they were still available from bike shops. I've had no complaints with it so far.
All of my riding has been strictly dirt and gravel roads. The only pavement I've ever ridden on was the short test ride the day I bought the bike.
Anyway, fast forward about 9 months of off and on riding and I am about 70 pounds lighter. Now that the weather is starting to change (can't ride in the mud) I'm ready to get back in the saddle and drop some more weight this summer. With any luck maybe some time next year I'll be able to post my own success story and pics in the weight loss thread.