That calculator is exactly what I needed, thanks!

Definitely not either, I'd like to move away from drill drives - as nice as it is to have a cheap, ready made drive, they're pretty problematic even in the capacity I've used them, and I've never even fought before...
I'm thinking things through with a baseplate too, not sure which'd be the most suitable material for the job within my budget - thinking of having a rather thick 6082 T6 (I think?) aluminium baseplate if weight allows, so hopefully that'll be suitable to mount a magnet to the underside of, or something along those lines - I've seen a few magnets designed with holes pre-drilled, so that might be a way to go. Not worked out specifics yet with the drive, but I'm going to try and have a magnet near each wheel individually if I can since that'll probably give the best results - might have a play around with some designs and see what works best on paper, based on that - thanks!