drilling out is simple, put a drill bit into a second drill, insert it into the end of the drill you are taking apart, and drill the reverse thread screw until its head comes off. you should now be able to unscrew the chuck. The wheels should be made with a 3/8ths UNF thread in them. screw wheels onto the drill shaft. lock in place either with a grub screw in the wheel or shaft, split pin, or a nut tightened up to the wheel.

I recommend the electronize speed controller if you plan to overvolt these motors, ( which both technobots and I sell). another option are mtroniks controllers, the marine 25s I sell being good.

remember you need to lock the torque rings too. i suggest m5 grub screws.

finally, 2x 9.6v drills on 12v should happily run a featherweight, as long as you have them balanced right, ie, do not have the motors at the back and all the robots weight on its wedge at the front. it will have trouble. 4 of these drills would be a really good drive system, but be aware that the harder you push them, the shorter the life of the plastic gears.