Thank you all for the advice, all of which makes sense. Currently I'm doing this on my own, I was going to tap another friend up to see if he wanted to join in but I've treated this as a solo thing just incase he says no.

While planning the robot I decided to get two 12v wiper motors from a transit van which have arrived today. With the talk of drill motors (which are smaller than wiper motors I think) I think I've already used 2kg of the featherweight limit!