Alex,
As soon as I read drills I thought, "too fast." So the rest of your post didn't surprise me.
Drill motors turn at a few thousand RPM and have about a 4:1 gear and then run at 450-1000RPM.
I suggest you use a moderately highly geared DC motor say 20 RPM and control the motors with pulse width modified dc motor speed controller circuits.
gearhead DC motor
http://www.ebay.com.au/itm/Metal-Rev...z1k-gIT21n5lIw
dc motor speed controller
http://www.ebay.com.au/itm/12V-40V-1...4AAOSwdIFXyP2F
cheers
Joe