I would think an entry level bike shop quality mountain bike or hybrid might be the best option for your needs. Most quality entry level bikes are really pretty good these days, you will get something that shifts and brakes nicely and is reliable. Your probably looking at $300.00-$600.00 for something like that. Craigslist is a good option for some people but if you don't know a lot about bikes a good bike shop should be able to help. Sometimes you can get good service plans as well. I think some places, Bicycle Village for example offer free lifetime tuneups and such. Also if you haven't noticed there is a sub-forum on bike forums Clydesdales/Athenas (200+ lb / 91+ kg) where the people will have more experience with your needs.