What is your ultimate goal? Are you trying to loose 100 lbs with a new exercise program? It is OK either way, yet the bike choices for a 300 lb person and a 200 lb person would be very different so you might save your budget till you get further along.
You're stuck in the MTB market (or possibly Cyclocross or Hybrid market), and probably with 32/36/40, or 48 spokes.
I wouldn't plan on any rough off-road riding, unless you go with a Fat Tire Bike which is another option.
If you think your weight & goals will be rapidly changing, I'd go either the Craigslist or Dept Store route. Also keep in mind that a lot of exercise equipment is prettier to look at than to actually use.