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?
Don't bother trying to make a speed controller. It's a bloody difficult job for people with decades of electrical experience with many blown up boards. For those without the electrical background it's near impossible.

Those bosch 400s whilst they seemed like a good deal at first have been a pain for people to use as they are difficult to find a suitable speed controller for.

With regards to the batteries, hold off on buying them. I have bought a couple of 2.7Ah nano turnigy batteries that I intend on running in boner at the next UK champs. I will be able to give you a proper run down on them after then.