Hi Andrew, If your not sure which drills to use keep in mind that most people who use drill motors and gearboxes use the 18V ones, the 12V or 18V from Maplin seem to be fine, but be careful of drills with Hammer Action as they are more complex and harder to adapt for robot use, same goes for 2 speed drills, as you will never use the low speed and its just wasted weight. The 550rpm ones need bigger wheels (6) for reasonable speed but some are 700rpm and smaller wheels can be used.