If you get a used one that has not been rebuilt I would try to replace as much as you reasonably can before having it reinstalled.
I would at least replace the carrier bearings/races, the pinion seal/bearing/race/crush sleeve, and ring gear bolts. All of this can be had for $125 or less for a good brand like Timkin, SKF, or National. I would also replace and the differential gasket, axle shaft seals (if you can get them), and the u-joint. These are not particularly expensive either.
Whenever you buy parts for a drivetrain you need to do some research since there can be multiple options. You need to confirm exactly which differential and axles you have. You need to do this before buying a used unit and also for replacement parts.
I would call around and let mechanics know that you have a used differential along with some parts that you want replaced before it's installed. Work with them to put together a part list, but let them know you will be buying the parts. A good shop will generally be ok with this as long as you are getting quality parts. There are probably more shops that will swap out a differential than will open it up and replace parts so you need to look into that. Transmission repair shops often do differential rebuilds as well so that's a place to look .
If it were me, I would also end up grinding off all of the rust from the case and repainting it before it went on, and I would replace all of the mounting bolts. I end up wasting more time than some as I want it to look nice for a while as well as work. I am always fighting rust up here so most things that come off get painted before they go back on.
LMHmedchem