My advice:
Figure out whether you got a long bed or short bed, step side or fleet side.
Listen to the other guys - post a "Wanted to buy" ad in the classifieds (if possible) and start first with a request to purchase a "clean" cab.
Once you get the cab, start your restoration. While you're restoring, you can start searching for a fleet side or step side bed (either short or long).
Some people sell cabs for a little more that have been patched up already to take out the very common rust spots on these trucks (cab supports, rocker panels, etc.). You got a couple options here:
1) Buy a cab that's already been patched up in those spots, then swap over your VIN plate to the new cab.
2) Buy a cab that needs to have these items patched and patch yourself with a welder and using "ebfabman's" videos (here:
http://ebfabman.com/)
3) Buy a cab that needs to have this work done and haul it to a Goodmark install location (call them:
http://www.goodmarkindustries.com/) and have them do the repairs, very reasonable pricing!
4) Buy a cab and have someone else do the work.
Those are pretty much listed in order of cheapest to most expensive. #1 and #2 are about tied, because the tools and panels you will need to buy for #2 will be pretty close to what you will pay for a pre-patched cab. The hard part is finding someone whose work you can trust on those repairs.