I have a Rheem 5 ton AC unit installed in May 1996. Model RAMA 060JAZ. It seems the compressor is 'kaputt' (gone in German), it has a short and trips the breaker. The compressor was acting funny lately. Can this compressor be replaced on site, or does the complete unit has to be replaced?

They can be replaced on site, but the unit is 11 years old and you should look into higher efficiency AC units and whether the likely R22 refrigerent system should be replaced with a system using R410A.

You can also opt for a duel-fuel type of system where you replace the AC with a Heat Pump and still have fossel fuel as well.