Should I paint this before installing, or can I bolt it up as-is?

The primed silver finish is durable enough for direct installation if you like the contrast look. If you want it body-color, sand the primed surface lightly and apply your topcoat before mounting. Either way works—it's your call on the final look.

What fasteners do I need to secure this to the bumper?

Hardware is not included with this trim piece. Check your BMW service manual or test-fit the molding first to confirm which fastener type and gauge the factory mounting points require—usually small clips or bolts depending on your exact trim configuration.

How do I prep the primed surface if I want to paint it?

Light sanding with fine-grit paper is standard before topcoat. Beyond that, follow your paint system's prep guidelines—primer-to-paint adhesion is usually straightforward on a primed plastic surface like this one.