Thanks for all the advice, guys, I really appreciate it.
@dotDominic That workmate thing looks excellent, I think I'll probably end up getting one of those. It'd certainly save me getting a workbench and a vice, at least for now.
I've taken a look at some of the build threads and I've seen that Australian Robot Wars video series. I'm fairly experienced with electronics and DIY (I study Mechatronic Engineering), but the basis of my design at the moment should be fairly easy to create anyway, and then I should be able to attach my lifter mechanism once the main chassis is up and running.

HDPE sounds like the way to go for featherweight armour, at least as a starting point. I assume it's good against impacts, but what's it like against cutters and rotating drums?