To sum it up, I doubt you will be able to find a controller for 4 drills for under 50 quid. Hate to say it but you're either looking at spending more cash or perhaps modding a couple electronize.

DONT TRY AND USE A SABRETOOTH WITH OVERVOLTED MOTORS

A very nice controller if you respect its specifications. Try and push its limits and you will most likely see the smoke.

Don't bother overvolting 12V drills to 24V, 18V should be fine but 24V and you will see smoke from them after a 3 minute battle. Not to mention that the machine will probably be too quick to control.