Bad AC Clutch, Compressor Too?

Recently while driving at high speeds (70+) I was experiencing a disturbing grinding/rubbing noise in the vicinity of the alternator/ac
I took a look while the engine was running and goosed the throttle, noticed the AC clutch was engaging/disengaging. While the AC clutch was engaging was when the awful noise was occuring, so I unplugged the AC wiring harness above the alternator, problem solved for a few months at least.
Would the clutch be engaging/disengaging for a low charge, or could there be other reasons? The AC worked last summer.
I am pretty sure the AC clutch is shot, so rather than just having this part fixed before summer, would it make sense to replace the compressor and dryer as well? Then there is an expansion valve that should be replaced? Where would that be? Just curious really. I don't have the tools or experience to pump the system down, check for leaks, recharge, etc.
Definatley a good opportunity to go to R134A, but given that this is a 1990 Escort GT I'd have to be settled on some other problems first before I spend that kind of money on this car.
Thanks, Matt
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