From your posts, I would like to steer you away from Craigs List or other independent sellers. While you can get some great deals, you have to know a bit about bikes in order to know what to look out for. Find a reputable LBS or two and see what they have in used bikes. A used mid-level mountain bike outfitted with hybrid (more like road tires than knobbies) tires would make a solid commuter. 36-spoke wheels with double wall rims (common on anything above entry level MTBs) will support your weight adequately. As mentioned, have the LBS check the tension on the spokes to make sure they are tight and even. Spokes aren't broken by the weight of the rider so much as they are from excessive play.

Flatbar bikes like MTBs and fitness/hybrids give you a more upright riding position which you might find more comfortable for now, but don't let drop bars scare you off either, they can be set higher with an appropriate stem.

If you want more of a road bike than a modified MTB, look at touring models like the Surly Long Haul Trucker or Trek 520. Both are sturdy steel frames with a relaxed geometry and are common enough that good used ones can be found. Within a couple of months the LBSs will be getting in lots of used bikes as trade ins, so the pickings should be good.

Good Luck