I"ve bought 4 engines from salvage sources. 2 came with 30 day warranty.
I borescope the cylinders. Pull a valve cover to look at the amount of sludge build up. Look for clues of abuse or hack maintenance.
The 2 engines with warranty; I installed them and drove it. Really didn't do much for prep.
3 of the engines were first run units with reported low mileage and look to be just that. I wasn't too concerned with them. I trust first run engines better than I do machine shops and I don't want to spend the money If I don't have too. Tear down and overhaul "just because" doesn't go well with me.
One of the engines was a "rebuilt" 318. I probably shouldn't have bought. It was a decent engine but, the cam was "wild".
The seller couldn't tell me the specs.