I am not an expert on AC units but this is what I do know. Freon also acts as a lubricant for the seals of the hoses and compressor. The seals will dry out if freon is not present in the system. If your AC system has been without freon for a long period of time be prepared to have to replace compressor and some hoses and hose seals (O-Rings). The mechanic is correct because pressure and vacuum are two different things. The ac system is a high pressure system. The only way to find leaks is with the dye because they will be able to see the leak when the system is charged. If the leak can be seen then you know what needs to be replaced.
If the compressor needs to be replaced just replace it with a good unit. I would recommend an AC Delco remanufactured unit. Don't skimp with the compressor. The price varies. If hoses need to be replaced consider having them rebuilt. By rebuilding they take the metal fittings and put on new hose and make sure it is sealed properly. As an example I had one bad hose (it leaked where the hose meets the metal). To replace it was $90, to have it rebuilt was $38. Big savings. I sent mine out through Napa Autoparts, I am not sure if you have Napa in your neck of the woods. You can also try the yellow pages under Air conditioning.
Finding R-12 is another issue. In AZ only mechanics can purchase it. Its an EPA thing. R134a you can buy all day long but not R12. R12 is expensive. I paid $4 per ounce and it took 2.8 pounds of the stuff to completely recharge my system after I replaced the compressor, accumulator and a couple of hoses.
I hope this helps.
