Since i'm in a quiet similar build status, i'll just jump on here, okay?

Would Polystyrol also be possible for building?
It's way cheaper (or i only found expensive sources for HDPE) and at least here more available.
Or would that shatter or get ripped to pieces on first contact?

And on the Batteries, for the combination of motor/gearbox from ranglebox suggested in this thread, how much capacity should the Battery have?
5s come in a lot different sizes/shapes/capacities, so... how much would you need, suggested you only use it for driving and possibly some minor stuff like a servo or such?