Do these headers need gaskets or sealant?

Use quality header gaskets at all cylinder head ports. Don't rely on sealant alone — gaskets prevent blow-by and last longer under heat.

Will I need new exhaust piping downstream?

Header outlet diameter isn't specified here — verify the flange size matches your existing downpipe or have a shop fab a transition if needed.

What about ceramic coating or polishing?

These ship as raw stainless. You can polish for show or leave them bare — stainless doesn't require coating to resist corrosion like mild steel does.