5mm steel is usable, but for the weight you're probably better off working with some sort of wear-plate, like Hardox. Same weight as steel but for combat it completely outclasses it. It's hard to find 3.2mm anymore, but 4mm Hardox will prevent pretty much anything from getting into your machine and it'll be worlds tougher than 5mm steel. That said there's the cost to balance, and 5mm steel will do the job, it just may get bent by some of the harder hitters.

As for brushless drive I don't think you're looking at huge cost for the reduction, really. Around the feather drive scale the rpm is pretty versatile. The typical 550 drill motor spins at around 20,000rpm. You can get that and lower with brushless fairly easily.

You won't be able to use completely unmodified innards from a drill gearbox, as it'll most likely shred itself under much more power than the normal 550 size, but if you don't drop in a huge motor and you take the right procedures I can sort of see that working. With an expectancy of short run time, anyway, haha.

An alternative, though it ramps up price a bit, is to mate a brushless to a Banebots gearbox.

Good luck anyway, will be interesting to see brushless drive in action.