I have bought a number of used engines over the years.  Here's my two cents.  Try to find an engine out of a 1957 4 door or station wagon.  All 57 YBlocks came with the good heads and rockers, distributor and exhaust manifolds.  1957 272s are great engines to buy.  They are more common, normally weren't hot rodded and you can bore them to 292 size. I have the original 292 block in my TBird.  The block is all that is left original. My original engine had been rebuilt so many times it was junk.  I bored my 292 .060 without any issues.  Still, it needed align boring and the decks squared. Whatever YBlock you buy, you will most likely have to do the same. Obviously, the less miles on the engine the better. Unfortunately, you can't tell the actual miles or how it was maintained from the outside.

I never buy someone else's rebuilt motor or any engine that has been hot rodded.  What you want is a pure virgin.  If it still has the 2 barrel carb and manifold, all the better.  My motor cost me about $1,500 for the machine work alone. I probably have about $5,000 US in it total counting the 4V intake manifold, holley carb, 57 distributor and exhaust manifolds.  Correct assembly is important, especially in the area around the rear seal.