My thoughts:
Not exactly hardware, but when you mentioned things that go out of stock quickly, I instantly thought about 775pros
Plenty of people and teams love that motor, and I am one of them!
I would say to have a nice supply of different gear stages for the VP so you can do quick swaps when prototyping or for the actual robot. Ring gear sets, output stages, all of that. If there's one thing I have learned by now, it's having all the things you know you are going to use on hand. Nobody has time during a build season to wait for an order of some sorts to come in.
The VersaFrame plethora of offerings seems to be quite great, yet I have never used any of it myself. Gussets, tubing, and mounts for all your VersaPlanetaries. The different kits they have these days are great again for rapid prototyping or robot assembly. More stuff to have on hand to make your life easier during build.
If you use 1"x2" box tube, 1"x1" box tube, or other sizes of whatever wall thicknesses, get your big orders in now. Maybe you like to use the VersaFrame tubing or maybe you do some custom stuff yourself, either way, order the standard tube sizes you know you will probably use anyways now.
Spacers and Shaft stock! Some of the ThunderHex stock that has been sold for a while now is slightly oversized. Buy it now and turn it down or sand it so that the shaft will actually fit nicely into a ThunderHex bearing. Again, another thing to do now instead of during the season. The acetal spacers in nominal lengths for 1/2" and 3/8" Hex has saved us plenty of time in the past. Invest in them for your own spacing needs.
Common fasteners. 10-32 or 1/4-20 fasteners. Maybe you used some #8 in there too. You know you'll need them. Oh, and something that you will probably end up using on your fasteners: Loctite or comparable thread locker.