Thanks for responses,
Pigggeh: I hope there will be plenty of wiring room as I expect the motors will require the chassis to be quite large (using cordless drills). I agree about the bearing point you raised. Should I place bearings on the top as well to protect against downward strikes like that of an axe?
Omnibot: I can't really comment due to lack of experience but CAD seems a good start.
Daveimi: I aim to complete stage 1 before attempting anything exotic. This will allow me to adapt, if its too complicated, into something like a bulldozer.
Thanks,