Hi Tom
Others will fill you in with regard to hydraulics.
For drive though the tried and tested route is brushed and there are a number of reliable configurations. However...

A few people are now putting brushless drive systems into HWs. I believe two were running brushless drives in the current series(hopefully I will be corrected to say there were more) our entry ran out of time finishing the last couple of weapon bulkheads, but we did finish and test the brushless drive and whilst its not geared for a high top speed it gets there with featherweight levels of acceleration. This uses scorpion hk5020s and our own speed controllers now known as Mangle VESCs (see for sale section for techy details) after Rory from team Nuts who did most of the development. This gives us 7400 watts of drive as opposed to most brushed drive systems which typically deliver 2-3000 watts peak. The other plus is the weight saving which is a small fraction of a brushed setup. We are also fitting a brushless drive to our new HW intended for non spinner competitions, currently under construction. This uses two brushless scooter motors with the same Mangle VESCs.

We have had teething problems as there is quite a bit of fine tuning needed in software but the system is definitely viable and as cheap or cheaper than a brushed setup, depending on which brushless motors you use.

Pm me if you are tempted to go down the brushless route.

Good luck

Ian