What prep work is needed before I paint this mirror?

Light scuff with 220-grit minimum to break the surface gloss, then clean off dust. Wipe with a tack cloth before priming. Use quality automotive primer rated for plastic substrates.

Will the power motors work right out of the box?

Test the power connection before final installation. Plug into the vehicle harness and cycle the adjustment motors through their full range. If motors respond, you're good to install after paint.

Do I need to remove the old mirror to install this one?

Yes. Remove fasteners at the factory mounting points, unplug the power connector, and swap in the new assembly. Installation takes about 15 minutes with basic hand tools.