Thank you very much Ocracoke, that is great advice! Really helped with some of my concerns.

Just thought I would give a quick update. After a few hours working on it each afternoon between the usual bank holiday family visits and drinking and all that, I've made some good (but slow) progress!

Drill motors are modified and wired in to the ESC. I've left the drill chucks on for now, as I can mount the wheels easily with bolts and electrical tape for a test run. I'll be modifying these further to remove the chuck and lock the gearbox, but for testing purposes it should be fine for now.

Had a slight delay in that my Sabertooth has screw-in connectors, so no servo cable to connect it to the receiver! Servo cables arrived today (thank you Amazon Prime). I'm using the drill battery for a test, but will definitely be replacing it with a LiPo before it goes in to competition. A family friend who is an RC helicopter enthusiast has lent me a balance charger, so I'm sorted on that front for the time being.

All that is needed now is to break out the soldering iron for a removable link, then link the transmitter to the receiver and it should be good to test! Fingers crossed!