Re: the HDPE. I've only ever had stuff from Direct Plastics, but the black HDPE definitely machines differently to the white natural HDPE.
I've not experienced any difference in machining black and white HDPE on The Honey Badger.

The motor and ESC look OK to me but it is important to note that the motor can technically run at a higher voltage (8S) over the ESC (max 7S LiPo) so I wouldn't exceed that. There is also the thing to note about saws (and rotational weapons in general) in the rules as well:

12.3 Rotating Weapons
The speed of any rotating weapons - e.g. circular saws, carbon or steel cutting discs - must not exceed the manufacturer's specification. The manufacturer's specification must be available for inspection.