Whole air conditioning unit needs to be replaced?

Would like to know how to tell if the whole air conditioning unit needs to be replaced or just the compressor, if that is possible? A couple months ago the a.C. Seemed to work at random.A couple weeks ago a loud grinding type noise started coming from the serpentine belt pulley attached to the a.C. And would get worse when the defrost or a.C. Was turned on. Looking down at that pulley it seemed "plate?" That it was attached to seemed to be a little loose from where it sits. Last night a sort of whirring noise started, sounded like it would tighten up and let go repetitively. Today the serpentine belt broke. Pretty sure due to the a.C./Compressor issues. Can the compressor be replaced without doing the whole a.C.