Re: So I saw this ad on Craigslist...
You've gotten some good advice here. My two cents input is that I'd say keep it simple and get riding. A cruiser makes a good motorbicycle that is pretty easy to set up and comfortable to ride. While you're enjoying your first motorbike experience give some more thought to that next bike. You can always sell your first one that you learned on. Ride somebodies chopper as a pedal bike to see if it seems to fit you. With a first, pretty simple build under your belt you'll have a better idea of what you want, what's involved and whether or not you have the skill and interest to do a more difficult build. Most of all, have fun and stay safe.
Someday when I grow up I will probably lose interest in toys with wheels, but until then...