Materials; Laser cutting

[emisnug]

So, I'm almost at the point in our design where I'm happy to send things off to be cut with a laser. It's a basic enough design, everything is sketched out where I want it (the construction lines on the image), I'm waiting on the motor mounting pattern before I send it off. The question now comes down to material.

By my reckoning, I've got 2 solid, cheapish options, and 2 expensive options.

Mild steel. Heavy, but cheap as anything.
5083 Aluminium. Slightly lighter, but still pretty cheap.
7075 Aluminium. The grade I actually want, but price quadruples over 5083.
500 Grade HMWPE - Prices vary; I've never worked with it before.

By reading through the build diary for Gabriel, I know that HMWPE can be used without too much issue, but I'm worried that it might not withstand impacts all that well. And I have no idea how well it holds a thread, which would be key to making sure the bot is stiff enough.

What would everyone's preference be? Any recommendations on laser cutting places?

A couple notes about the design:

Two frame sections, held together by screwed in crossmembers, with a total height (not including armour) of 180mm. Each frame section is 25mm in depth (if steel, 12.5mm); the construction lines show drive motor position and mounting, wheel position and battery location. I'm still working out the joining functions on Inventor, so unfortunately no isometric models for now. If there's anything glaringly wrong, please do let me know.

Big Dave 3 Frame MK1.PNG