$300 is on the high range for a drum brake rear axle. setup. I see them all the time for 2-300. The problem is you don't know if you are buying a 40000 or 200000 mile rear end, or why it was taken out- was it an upgrade to discs brakes or because the posi unit is bad, leaking or twisted axles, noisy gears, etc. Think of your average 5.0 owner and what they do to these cars over 20 years vs. what you already know about your car.
Like what was said, pull the cover off. Smell the fluid, look for chipped gears, shavings from the clutch packs or shims. Bring some playdo or clay and put some on the ring gear and run the gears to see the contact patch. You want to see it deep and in the middle of the ring gear. Also make sure the housing/axle tubes aren't cracked or rusted out.
If you plan on using the brake hardware- are you planning on transferring over your brake shoes, springs, drums? Does it come with the E brake cable, brake lines/hoses?
Pull off the drums and inspect the wheel cylinders for leaks - I would replace them anyway now that they are off the car
Look for any fluid on the shoes
Inspect the drums for scoring, cracks, warpage
make sure all mounting tabs are intact
Inspect the pinion flange area for leaks and that it turns proper
If you can
Pull the C clips off and pull out the axles. Inspect the bearings and seals.
inspect the axles themselves for cracks. It will be hard to tell if they are twisted from abuse.
At a minimum, I would install new brake cylinders, new axle seals, if your rear brake pads, springs and drums are good transfer them over, and clean and refill the rear end with fluid and friction modifier, and a new set of U joints. The right way would be to install all new brake hardware- cylinders, springs, pads and turn either turn one of the set of drums or new drums if they are not in spec, and new axle seals and bearings. No sense having to pull everything apart twice. You can get most of the parts from Autozone, Rock Auto, LRMS, etc.
So IMO to do it right will cost more than $300. Probably another $100 or so for parts but assuming everything else is good you'll be all set.
While you are at it, inspect your UCA and
LCA bushings, rear shocks, etc. Again, no sense having to take stuff apart twice.