Do I need hardware to install this, or does it come with bolts?

This assembly mounts at factory points, but fasteners are not confirmed as included. Grab your service manual or do a test-fit to confirm what bolts you already have on hand — odds are you can reuse them.

What do I do before painting?

Light scuff with sandpaper to rough up the primer, then wipe down with a tack cloth to remove dust. Primer already on it, so you can go straight to base coat and clear — no additional prep needed.

Will this guard protect against actual bumps, or is it just cosmetic?

It's rated to absorb low-speed impacts and shield your rear bumper from parking lot dings and road debris. Heavy collisions are beyond its scope — it's designed to take the hit so your actual bumper doesn't have to.