hands on im pretty competent that way, but as for knowledge on the motors its a no no. Ive only at this moment measured the shaft with a tape, so far from accurate but its it around the 6-7mm mark, i presume when you mention the single stage gear that would be a (sorry for my terminology) a small sprocket on the drill shaft to a larger sprocket on the wheels?

Would you advise for someone like myself who is just starting out to run for example 4no cheap 12v drill motors over volted (i have 2no already) and just run them trough a set of bot bitz 85A esc's?