You could always build your own linear actuator. Attach a drill motor to a threaded rod and run a nut along it.

Yeah you can run all your power from a single set of batteries. Makes wiring up a lot easier.

Robochallenge wheels are going to be your easiest bet

Speed Controllers are best just bought. You can try and make your own but it's a bigger pain in the proverbial than it's worth. There are a number available. Everyone has their preference. I like the series that electronize does. Nice and robust.

Is the machine for show or for actual combat? I'd recommend dropping the 1mm steel shell and using some HDPE. Better at absorbing impacts and far easier to work with as it can be machined with wood working tooling.