Do I need fasteners to install this, or does it come with hardware?

This ships as the diffuser panel only. Source fasteners rated for your vehicle's rear bumper mounting pattern and torque requirements — refer to your vehicle's service manual for proper fastener specifications.

What prep is needed before painting?

Sand the primer surface with appropriate grit to promote adhesion, then clean thoroughly to remove all dust and residue. Your chosen paint system (urethane, acrylic, or equivalent automotive finish) will bond best to a clean, lightly abraded surface.

Will this fit with my current rear bumper, or do I need to modify anything?

This diffuser is designed to use factory rear bumper mounting locations, so no cutting or drilling of the bumper itself is required. Verify your bumper's hole positions match the diffuser mounting tabs before final installation.