For non running engines:
You can usually tell by pulling the valve cover and water pump or thermostat how badly a block has been abused- If there was no oil to the top end the rocker arms will be pretty worn out and gummed up. You can also look for signs of bad valve guides and seals. Pull out the spark plugs and make sure they are all the same light tan color- if you see white ash or a bunch of oil sludge on one or two plugs there is a problem in that cylinder.
Pull the distributor cap off and give the rotor a twist to make sure it springs back (checks the advance weights) and check the breaker cam for wear. This should give you a good idea how many miles the engine REALLY has on it.
I wouldn't bother to pull the head on an engine you might buy- most sellers will take offense to that. Just pulling the valve cover and replacing it doesn't require a new gasket set.
Expect to replace a few gaskets, the fuel and water pump, and maybe the valve stem seals on an engine that has been sitting up for a while. They harden up when not used and leak once the engine is put back into service.
