Do I need to replace the downpipe and cat, or can I use my existing exhaust?

The header bolts to stock mounting points, so your current downpipe will bolt directly to the header outlet. No changes needed there unless you want to upgrade the whole system for maximum flow.

Will this header fit with my stock air intake and exhaust manifold heat shield?

Test-fit before final assembly. Tube diameter and runner routing may require clearance verification with your specific intake and accessory setup. Minor shield bending or repositioning is common.

Do I need a gasket kit, or does anything come with this?

The header assembly ships alone. You'll need exhaust manifold gaskets (available separately) and header bolts to complete the install. Grab a gasket set before you start tearing into it.