Take tons and tons of pictures, pictures of everything, even things that don't make sense to take pictures of. Later, you'll be looking for a picture of something that you wish you'd taken a picture of, and it will show up in one of these other pictures. Bag and tag everything. Put small parts in bags, and put multiple bags for assemblies into bigger bags. Put these in boxes, and keep everything in the same area. Take pictures of EVERYTHING before and as you're disassembling it. You will NOT look at these pictures later and be mad that you took too many.

As for parts to buy, you need to come up with your plan for the big operations, and then decide what parts you need for these. You should keep an eye out for key, hard-to-find things as the other poster said, and buy these as soon as you find them. Everything can be sold later if you don't need it. Other things, which you can buy at retail, you can hold off on to an extent, but be aware that some parts take time to get even at retail, so make sure they're not on backorder.

Probably not a bad idea to hold off on the engine and trans. These are things that can usually be done in an anticipated amount of time. Other things, like body work and/or paint may not be so easily anticipated. My engine only sat around for a year and a half, but no problems. It was fogged and bagged. It WAS broken in prior to this.

Good to get stuff off that needs to be stripped, and get this turned around and then painted, then they'll be ready when you need them.

Finally, I think someone else said it before, but make sure you do your math for the cost of the project. Take what you're anticipating it to cost, and multiply this by 2.5. That's about where it will end up.