Will I need a new gasket or sealing material to install this?

You'll want a fresh gasket set for the manifold-to-head interface — reusing old gaskets on headers often leads to leaks. Grab a new gasket while you're sourcing the header.

Does this work with my stock exhaust and downpipe, or do I need to upgrade those too?

It bolts to your existing downpipe and muffler setup, but you may notice turbo lag or boost loss if your turbo and piping are still severely restrictive — the header alone won't unlock full potential on heavily modified builds.

Should I paint or coat the outside before installation?

Stainless won't rust, but it will discolor under extreme heat. A high-temp coating is optional if you want to preserve the finish — test-fit first to make sure clearance is solid before committing to any coating.