I'm not sure on a speed controller yet-
I will hopefully try to build by own with the help of our electronics technician at school, but if that doesn't work I will have to try and buy a second hand one.

I will only overvolt them to 18.5v making them comparable to a Bosch 750, I will try to use a low gear ratio to reduce the load on the motors to stop them drawing too much.

Could I use some kind of voltage limiter to set a limit on the current a motor can draw?