Q: I have no oil pressure (PSI) diagnostic says to replace engine. Which would be the best way to go

You say the only thing you hear is the knocking in the lower end, but this knock is a major problem. When you drove the car with no oil pressure, it damaged the crankshaft main bearings, and most likely the crankshaft and other engine bearings as well. I suggest having a remanufactured engine that has a warranty installed as opposed to a used motor. The problem with a used engine is you cannot tell how it was taken care of, and as you said how many miles it really has. When buying the remanufactured engine, make sure you get one with a warranty that covers parts and labor to replace it, in case the new motor fails at some point. Only buy remanufactured engines from reputable sources that have known good warranties. You also have the option to have your engine rebuilt, though this may cost the most depending on parts involved. The upside here would be that the car would still have the original motor. If you would like a second opinion on the matter, you could contact a qualified technician, such as one from YourMechanic, to diagnose the noise you’re hearing.