You can often get cordless drills dead cheap by going to a boot fair and looking around. Most will have their battery in very poor condition (try out the drill - see how fast it spins). Bargain with the seller. A crap battery makes a drill almost useless for work but it makes no difference to a robot builder since you aren't going to use the battery. Ask your friends - someone might have an old drill lying around that they no longer use because of the battery You can buy wheels that screw directly onto the output shaft of the gearbox, once you get the chuck off, elsewhere on this forum. Good luck!