I haven't begun any building yet. But I am in the process of much research. I have been planning on using the DeWalt 24v Hammer Drill motors with matching 2 speed transmission. it gives a speed option or a high torque option. They can be found at
www.robotcombat.com for one motor and transmission it runs approx 85 dollars. not including shipping and handling. Any opinions from the experts would be great.