Do I need the left side trim piece too?

This is the right side molding only. If you want the full bumper trim look, you'll need to order the left side separately. Most buyers get both sides for symmetry.

Will this fit if my lower valance has been replaced?

This trim mounts at the factory clip points on the front bumper itself, not the valance. If your bumper is OEM or stock-profile, it'll line up. Aftermarket bumpers or significantly modified lower sections may need test-fitting first.

Does the chrome come ready to install, or does it need prep?

The trim ships with the chrome finish applied. No painting or sanding needed — just clean the mounting area and clip it on at the factory attachment points.