Best advice for stocking up on supplies would be to look at what you used this past year and go off of that. Good thing there's a Bill of Materials to get a good idea
As for raw stock items from a mechanical side of things, I'd suggest standard stock items, such as 80/20 aluminum, aluminum box, C, and L channel, Lexan, PVC pipe and PVC Lumber (my team has found it to be a very useful material), bolts/nuts, and standard axle sizes.
These are the things you should order now and look for good prices and not worry about shipping, you're almost guaranteed to find a use for this stuff. Even if you don't, it's better to have something you don't need than need something you don't have!
One piece of advice for a team still kinda green
Standardize the materials you use! My team uses 1/4-20 bolts and 7/16ths nuts for practically everything that needs a bolt/nut, this saves a ton of time and effort looking for the right drill bit/wrench.
Hope this helps, and good luck in your 2nd season!