Will this headlight plug directly into my factory wiring?

These are designed as OE-style replacements, so factory connectors should align. Test the fitment before final installation to confirm your specific vehicle's harness matches the assembly.

Do I need the left headlight too, or can I just replace the right side?

You can replace just the right side. This is a single-unit assembly. If you want matching housings on both sides, you'll need to purchase the left headlight separately.

What mounting hardware comes with this?

The assembly ships as a complete headlight unit. Confirm your vehicle's existing mounting brackets and fasteners are in good condition before installation — hardware for attachment to the vehicle frame is not included with this assembly.