Will this bolt directly to my Focus without any cutting or drilling?

Yes. This assembly mounts at factory rocker points — no modification needed. Line it up with the existing holes and secure with standard fasteners.

Do I need to replace both headlights or can I install just the left side?

You can run a single left-side replacement. For best appearance and even light output across the front, a matched pair is recommended, but it's not required to make this unit functional.

What's included in the box — does it come with a bulb and wiring harness?

The assembly ships as shown — headlight only. You'll transfer your existing bulb and connector from the old unit, or source OE-spec bulbs separately if needed.