/6 cost

To Address your current problems. I dont think your mechanic was worth his salt. I would suspect your "head gasket leaking oil" is most likely the spark plug tube seals are bad. We can most likely talk you through diagnosing your problems and repairing them your self.
If you are bent on having the engine replaced expect a good shop to charge between 3 and 5 K for a good warranted rebuild or "new"Rebuilt engine.
Frank