I think you'd need something around 10kg righting force, with 20cm radius wheel that's 2kgm (= 2000gm = 200000gcm) That's a factor of 500 greater than that motor (even with reduction gearing it looks way way out).
[ torque = force * distance from axis ]
Too much force =/
Please look herehttp://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=290566655212&ssPageName=STRK:MEWAX:IT
I really wanted to do this project, these engines seem pretty strong and has a good price, but no way really? =/
Yeah - the kind of motors you are going to want to use will likely be similar to the ones seen on electric mobility chairs, or maybe electric scooters. That, and you're going to need one heck of an h-bridge controller to run it (which will be anything -but- cheap). You are probably looking at $300-500.00 USD just for motors and controller alone (and another $200.00 USD for batteries); that would be if you can get everything used...
Also this is anything but a beginner project; those kinds of motors have big-time current draw and lots of torque - without proper precaution and knowledge, they can lead to injury (possibly serious - possibly fire).
I was hoping would solve that with these engines. I had seen wheels motors, but they are really expensive. $ 500 is far from my budget.