Does this cover have sensor holes for parking sensors?

This trim piece is a lower valance—it sits below where bumper sensors typically mount. If your W212 has front parking sensors, they're in the main bumper cover above this trim. This part doesn't include sensor cutouts.

Will this bolt directly to my bumper without modification?

This piece mounts at factory rocker and valance points on the W212. No drilling required if the holes are clean. If you're replacing a damaged trim, check that the mounting ears on your bumper are intact before ordering.

Do I need the right-hand side trim too for a complete look?

This is LH only. Most owners run matching LH and RH trim for symmetry on the lower front end. You'll want the RH piece to complete the set—check separately.