Do I need fasteners to install these, or does everything come with the assembly?

The motors and regulators themselves mount to factory attachment points in the door. Fastener spec depends on your door panel configuration — verify what you have before disassembly so you're not left hanging mid-install.

Can I install just one side, or do I need both at once?

You can install one at a time. The set includes both sides because they're priced better together, but there's no requirement to do them simultaneously.

What's the return window if one arrives damaged or doesn't fit?

Contact ModSupply customer service with photos of the damage or fit issue. They'll walk you through the return process — don't assume you're stuck with it.