Twelve volt speed controllers, used in model cars, are easy to get. Msonic do line of these and there are lots of others. Try Sussex Model Centre. So 9.6 volt motors, overvolted to 12V, should give you good performance. Have a look at cost and availability of speed controllers and take that into consideration in selecting your drill motor/gearboxes. Gimson robotics do a 2 channel speed controller which goes up to 24 volts. Running your motors at 1.5 times their rated voltage will give you better performance.