Has anyone had experience with a stuffed aircon?

Dads GQ aircon hasn't worked forever and I decided to get it checked out and repaired.
Auto aircon joint called me and pretty much told me what I expected, it is completely cactus. He asked if I wanted him to stop and just leave it or keep going. Which gives me the impression that it is pretty bad.

Worst case scenario. What can I expect to be paying to fixing/replacing a completely stuffed aircon?

Would it be difficult to source components myself to use to have it fixed?

Do different engines have different aircon set ups?

Cheers

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