Bill listed most of the popular hand tools that would cover the majority of any scratchbuilding you'd want to do. If you find yourself needing more, then the move up to a mini mill/lathe would be a next step, but that would be a ways off. As for plastic stock, again Bill hit the popular sheet and rod sizes but I've amassed quite a selection of Evergreen/Plastruct stock in all shapes and sizes. Keeping them organized to be able to find them quickly can be a pain however. It's nice to have what you need at hand when a brainstorm comes over you when building, rather than having to wait for a trip to your LHS or online order to arrive. With my fading memory I'd often forget what I needed the particular stock for when I finally did get it