Find a used mountain bike without a suspension of any kind. I believe the Trek 7100 has a suspension fork, which isn't good for a person of your weight.
Once you find a mountain bike you might consider getting slick tires to replace the mountain bike tires. There are a number of 1980s and 1990s vintage mountain bikes that would work well for you.
If you need help trying to figure out what size let us know how tall you are. And even better, what is your cycling inseam (from crotch to floor).
Your craigslist has this Giant Rincon for $85.
http://daytona.craigslist.org/bik/3587306868.html I don't know if it would fit you, but something like this bike would work.