RPM depends on how fast you want to go. Just do the normal calculations. You can use JVN's spreadsheet.
The simplest gearbox motor combo would be direct drive with AndyMark Nano's and CIM motors.
Nano
AndyMark is also a great resource for using Mecanum wheels. Call them and get direct advise. It's summer they're just sitting around
http://www.chiefdelphi.com/media/papers/2755
Sounds like fun, please keep us updated with your results.