On my rig I had to replace two dryers (@ $20 each online) and the compressor ($200+ online) and the 134a (8lbs).
I could have just replaced the AC Clutch on the 17 year old compressor and not opened the system. But I decided to go the whole route due to its age.
If you do not have the rear OTR Bus Air option that cools the bedroom from the engine compressor, you will likely need less 134a and only one dryer.
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs