I have elderly in-laws in Outerbanks that have this Ruud UPKA-031JAZ unit connected to a Ruud UBHA 17J11NFDAI and RCBA-3765 GG17 Unit in their attic. They have no A/C and no Heat from the unit which they have been advised needed a condenser which was replaced and now have been advised that it needs a compressor. They do not have a lot of extra money on fixed SS income and I don't want to see them nickeled and dimed to death on this unit. Is it better to just keep replacing parts or should they be looking for another outdoor unit? What type of unit can they purchase used or new to connect to the indoor unit with respect to the new refrigerants today?

The company that sold them the condenser should have replaced the entire system. Sometimes limited financial considerations create complications.
At this point, you should look hard for a r22 outside unit. Most service companies say that these are not available but it is possible to locate them in some areas.
I am not familiar with the ruud nomenclature so I don't know the tonnage of the existing unit.