After a system is opened for service it must ALWAYS be evacuated, and THOROUGHLY so before charging.

The question would be whether or not the system needs flushing. Was the compressor only leaking, but working okay and without noise? If so, then you will not need to flush the system. Since it was originally R12 and if you know for sure that it has never had any other refrigerant then you are golden. Simply replace the compressor and r/d, add a few ounces of mineral oil, evacuate and charge.

I have used a few compressors bought on ebay with good success. Most of the MB ones can be had as a kit that includes the new r/d in the package.