Do these bolt directly to the factory door panel holes?

Yes, these mount at factory attachment points with no drilling or modification. Line up the brackets and bolt them down.

Will I need new bolts or fasteners to install these?

Test-fit the assembly against your door first. Factory bolts typically work, but bring extras to the job in case any are stripped or missing.

Can I replace just one side or do I have to buy the pair?

This set comes with both left and right. If you only need one side, contact support about single-side availability.