Ah yeah, I remember reading in the other thread that the 85s would be out of stock for a while. Fair enough, 30s will do the job fine.

As for the chassis material.... i'm a bit embarrassed to say that i've not really thought that far ahead yet.
Nothing to be embarrassed about there. A lot of people started out by strapping their components down to a bit of wood or chopping board as their first steps. If anything, it's actually better doing it that way as you can get familiar with all the components without having to worry about fitting them into a particular chassis or having to get the wiring really tidy. Just get used to what all the components are, how they talk to each other and have fun with it.

With regards to armour etc, the best thing I've found is to have a steel frame or chassis to form the main part as this gives a strong core, and then add a layer of HDPE as armour on the outside. The HDPE absorbs the impacts from spinners and other robots and dissipates part of it while the steel underneath keeps the shape and holds everything together. You can of course have metal as armour too but it is possible that the metal deforms from a heavy impact from a spinner and locks up a wheel or twists the chassis as the force tends to be transmitted through the chassis rather than just being absorbed and dissipated.

That's not to say that's the only way to do it though, HDPE can be used for chassis parts too. For example, I use 20mm thick sheet as the main bulkheads in my electric axe featherweight. As long as it's not a major load-bearing part or something that will be subjected to large forces, it's fine. You can also use Nylon as well for such things, but it is more expensive and not as easy to work with as HDPE, though it is stronger for things like bulkheads.

All of this you'll learn as you go along, so don't take what I say as gospel. It does of course depend on what tools and skills you have too for working on various materials, but to begin with, just do what you want to do and what's within your means, then go onwards and upwards from there