94 Nissan Sentra, Help! Do I really need a new engine?

Ok, so I have kinda a weird situation. I live on an island in the carribean and so the mechanical work is sometimes hard to trust and parts are hard to find. My car had been overheating and I took it to my mechanic who said that the radiator was full of oil so he drained it and "fixed some seals" well that happened about three times and he said finally that he thinks there might be a crack in a tray in the radiator so if it happens again I might need to take it to a radiator specialist guy. Well, it happened again and I decided to get a second opinion and that mechanic said that the problem is my cyclinder head(model GA14). And of course he can't find one on island so suggests that I buy a used engine and replace it, the one he found 555.00 USD. Someone please help I have no idea what is going on and I don't want to get ripped off because obviously I'm living off of loans for school right now. =(