Do I need a gasket or seal between the header and manifold flange?

Yes. Use a quality header gasket at the engine block connection point. Most builders go with a composite or metal gasket depending on your setup—check your specific engine configuration for the right fit.

Can I run this header with a stock exhaust system, or do I need a cat-back?

This header bolts to factory mounting points, so it'll connect to your existing exhaust downstream. Your current cat-back will work fine, though pairing it with a full exhaust system will let you feel the real benefit of the header's flow improvements.

Should I paint or coat this header before installation?

Stainless steel won't rust, so coating is optional—pure cosmetic choice. If you want a specific finish, ceramic coating or high-temp paint works. Install first, then paint if needed; heat cycling can affect adhesion if you coat beforehand.