Make sure it comes with the special shouldered bolts that attach the winch itself to the frame extensions on both sides. They are big $$$ if you need to buy them seperately. I think there are 6 or 8 of them total.
All the bolts that bolt the extensions to the frame are nice too, but easily gotten at a good hardware store, as are the 6 studs that hold the PTO on the tranny. make sure the PTO lkever comes with the metal mount.
The little safety catch/door thing that screws to the floor that keeps the PTO lever from accidentally engaging is a good thing to have. (the dataplate also that goes next to it is a plus if it doesnt get destroyed in the removal process.
As for a red flag, look for obvious abuse. Bent metal is a good indicator. Make sure the drum turns when you turn the input shaft, and the engagement lever slides in and out relatively easily. The drum lock should be free, not rusted in place. Also ask why is he removing it? Parting out the truck or scraping it, or ?????
Check TM 9-2320-361-24P, figure 165 and the next few for the parts you need.
http://www.jatonkam35s.com/DeuceTechnicalManuals/TM9-2320-361-24P.pdf