How big's your budget?
Post some tech info on proposed bot regarding batteries , wheel diameter , estimated wieght etc
Teams are Very cagey regarding the specs of certain items ie Motors
I was lucky enough to see the british bot "Panic Attack" up close a few years ago at a science fare in Cheltenham , and was politely refused a closer look at the electronics. Though the stickers inside would suggest that RS [RSwww.com] componenets are to be found inside....
Automotive starter motors have been used and are designed to produce large amounts of torque but require huge supply current (80+amps,will flatten battery pack in a couple of minutes)
Overunning is common, 24v into 12v motor, big performance boost but at great risk and does brushes no good (risk of failure during match)
Cordless drills are another solution , one for each wheel, they often have an internal gearbox , very usefull for direct drive application