I have to agree with Andy, most of the weight is over the weapon end and the wheels will not have much grip. The problem with adding belt driven wheels at the front is that you need two sets if the bot is going to drive inverted. Its not impossible but the long triangular belt path takes up extra space and adds weight. The other possibility is to try non-driven wheels at the front to reduce friction - I'm not sure if they would be effective though.

When I Google 'UK silver steel rod' some of the suppliers list hardened rod - a bit of searching will turn up what you need.

With the disk, I would definitely recommend some sort of hub,rather than just spokes that attach to the pulleys at their inner ends. without a hub, the screws will be taking a huge amount of force and might shear if you don't use a large size. With the right hub and pulley arrangement, the screws will only be subject to the force from the motors spinning up the disk. I will knock up a quick sketch of a typical arrangement later.

HDPE is a thermoplastic and can be bent when it gets hot enough. I have bent 5mm and 10mm HDPE using a specialised heat gun but 20mm might be harder.