The problem is, if you buy a used engine, you don't know 100% what your getting. Was the oil only changed twice? Did they overheat it to the point that it didn't cause any "permenant damage" but something is off just enough to cause an issue in 20k miles? The point is, if you buy a used a motor, you may end up having to rebuild it anyway. I don't know your budget but from my experience, a "good" used motor is about $800(up this way) and if the scrap yard offers a warranty on it, you have to do some work before starting it. I bought a used motor once and had to do the following for the 30 day warranty to be valid;
1. Cap, rotor, wires, plugs
2. Head gasket
3. Oil change(obvious either way but on the list)
4. Certain other gaskets
5. And there are about 5 or 6 other things I can't even think of right now but it wasn't a cheap list.
My recommendation, if you have a good foundation, work with that. Pull it, flush it, decide what you want to change and do it. If you get brazen, some new pistons, rings and rods, maybe have the head checked (only 50 to have it tested) and some new externals(radiator, high mileage oil filter, so on). Either way, definitely worth the rebuild.
Happy chevy-ing!