I would look for a decent used mid level mountain bike or "hybrid" bike. Something with a strong steel or steel alloy frame. As long as your frame is strong, the rest of your components can be changed as you have the budget and inclination. If you are into cruisers you may want to wait for some other opinions, I use a 26 inch MT bike and love it.
As far as parts go the, I have found that the local hardware store has most of the basics to make most of the stuff found on this site, I will admit that some of my stuff, like my engine mounts, are not as pretty as the machined ones some fellas make and sell, but 57 cents for electrical box covers and a few bolts later I have a decent looking imitation motor mounts.