well, ive bought 6pcs of DeuZer cheap 18v drills 20‚/each,(from finland ) we had 2 of them with our first bot running at 12v and 100mm wheels and now building new bot with four of them and 150mm wheels, the motors are running at 24v and each of them have own 80x80x25mm DC-fan and ill put some heatsinks for them if i can find any (i think wont find, ill make them from 1mm aluminium sheet)

Those motors work well at 12v, i sat to my new 4-wheel bot (i weight 54kg and the bot is 25kg) and it took me around easily and quickly