A modern mechanical cam and springs requires what work on the heads?

You need to look carefully. Some companies spec dual springs rather than single-with-dampener because of the faster ramps on modern cams. The duals need the seats cut and the guides cut. Depending on the lift you may need to shorten the guides as Skrews says. It's not a "must do" in some cases - but you have to make sure you understand when you're looking at the cams.

edit: - you may also save some $$ by running a beehive spring. Then it's just the guides and not the spring seats too.