I can't say I agree that two drills on a single TZ85a is overkill. I wouldn't go smaller, especially if slightly overvolting. When a gearbox goes and a motor stalls the speedo will just eat it up, which is what you want, the alternative is a dead speedo (much more expensive to replace). They will handle future more powerful drives too. It's cheaper in every way in the long run to get the slightly pricier speedos now.

Polycarb can be used although unlike HDPE it tends to shatter under impact, rather than just gouge/flex. You can mount it loosely or shock mount it to help its brittleness. Less popular now but everywhere a few years ago - it's down to design.

A steel chassis is possible, although yes, heavy. You would need to plan ahead (do some weight calculations, either with rule of thumb or CAD) to make sure you stay under, can't say without knowing the design if you'd be over. A combination of steel and plastics is seen all over the world as a compromise. Steel chassis with rubber-mounted polycarb armour (that can be replaced) would be pretty tough!