Do I need a new catalytic converter or downpipe?

No — this system bolts directly to your factory cat. You're only replacing everything from the cat back, so your existing emissions setup stays intact.

What fasteners do I need to complete the install?

Mounting hardware at the cat connection point is not confirmed in the product specs. Before installation, verify what fasteners your vehicle uses at that junction and source them separately if needed.

Will the polished tip stay shiny or does it need clear coat?

Stainless steel naturally resists tarnishing, but road salt and moisture will dull the finish over time. A clear coat or ceramic coating will extend the polished look significantly.