I guess there are six main areas for the drive....

Motors - Voltage, current draw, weight
Gearing / Transmission - Top speed, acceleration, strength
Electronic speed controller - RC interface, current and voltage limits, matched to motors.
Batteries - See some of the battery threads, Lipos seem to be the best but a little tricky to handle. Enough capacity to last a battle, enough current to power the motors at stall.
Wheels - Lot's!
Chassis - Lots of impact on the drive, wheels forward so the robot turns to face others quickly, to the rear for better weight transfer etc. etc.

Each is a large subject with many opinions (grin). The advantage of buying an off the shelf motor / gearbox / wheel set is that, if it has been done well, then everything should be matched and ready to go, the disadvantage is that they tend to be expensive and not the most powerful.

One of the areas to consider is how you want your robot to perform, acceleration, turning, top speed. Then do some math and it may help you to decide?!?