I just bought some wheel studs and spacers on eBay. What should I have known before I bought them?

-I am fairly certain that I want 20mm spacers for the particular wheel I have.
993.362.140.04 – 10Jx18 ET65 (Rear)
-I will probably be regularly swapping wheels between those and another set of stock 18s and 17s for street and track.
The studs I was not so sure of. They are 90mm and came with the spacers. As a set, I expect the length is good. What should I have looked for as far as material, length, and other specs?
Thanks.