Do I need to use any gasket or sealant where this connects to the manifold?

You'll want a quality header gasket rated for your engine size. Most people grab a fresh OEM gasket or aftermarket equivalent when bolting in headers — don't reuse the old one. Whether you use sealant depends on the gasket type; copper-based gaskets usually don't need it, but follow the gasket manufacturer's guidance.

Will this bolt directly to my stock downpipe and exhaust?

This header connects at the engine flange. Your stock downpipe should bolt to the outlet, but test-fit before fully tightening everything — alignment can vary slightly depending on engine bay condition and any prior work done to the car.

Does the stainless steel need any prep or coating before installation?

Stainless comes ready to install. Inside the tubes may show factory residue, so flush it out if you want clean flow. Some builders wrap headers for heat management — that's optional. No painting or chemical prep needed before bolting on.