To answer the HDPE question - before I joined a team and started working on a heavyweight, I found a lot of HDPE providers on eBay that cut to your specification and then ship it out to you either free or at a pretty reasonable cost, just type 'HDPE' into eBay and marvel at how many sellers there are! If you have the tools to cut it at home, you can just order a huge few metre sheet then draw on patterns for your robot and cut them out yourself.

I think we'll also end up using HDPE for the base of our bot for easy access for repairs etc.