Do I need to use new gaskets when installing this header?

Yes. Replace the factory header gaskets before bolting on the new assembly. Reused gaskets often leak, which defeats the whole point of upgrading.

Will this header work with a stock exhaust system?

It bolts to factory mounting points and flows to the stock manifold location, so it'll work. You'll see better results pairing it with a cat-back system, but it's not required.

Should I paint or ceramic-coat this before installation?

Bare stainless stays shiny and doesn't require prep. If you prefer high-temp coating for heat management, apply it before install — the header gets too hot to work on safely once mounted.