Well the ideal option for you would be a 24x3 3 speed coaster, which really could just be a 24 inch wheel that you can fit the 3 inch wide tire on. The shimano hubs retail for $60, which I'd definantly go for instead of a flip-flop if I were going to have it laced in.
I mean I guess that it's possible to set up a derailer, you'd have spacing issues though and it would be tougher with the dropouts, I know that one company did when the mountain bike craze hit.