The wheels and motor for our robot were taken from a RC car. This was chosen because the motor was already connected to drive both rear wheels.
Additionally the front wheels were already attached to hinged mounts, allowing for easy connection to the servomotor.
Finally the motor was known to be powerful enough to drive an model car at high speed.
The motor is controlled by an full H-bridge mounted on the PCB. It is capable of 0-100% power in both forward and reverse.