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

The trim piece mounts at factory points. Fastener type and quantity aren't confirmed—test-fit first, then source hardware based on your mounting method (bolts, clips, or adhesive backup).

Does this need paint or primer before installation?

Finish condition isn't specified in the product data. Inspect upon arrival and prep as needed before install to prevent corrosion at mounting edges.

Is this left side only, or do I need to buy two for both sides?

This is the left (driver's side) deflector only. Order a matching right-side trim separately if you want full front coverage.