The OE system used R-12 refrigerant. Currently not in production and $$$ if you can find it. Current refrigerant used in autos is mostly 134-A. This is not compatible with R-12 systems. Generally the compressor and drier. Hoses and seals, charging ports . If the system was in functional shape. If the a/c system was left open exposing the lines, condenser, evaporator etc.. The entire system, less the hoses is aluminum which oxidizes when exposed to the atmosphere, you may need even more. The systems are designed to use different types of oil, hoses and seals. They operate at different pressures and they are not equal in cooling efficiency. In most states it requires a license to buy and service a/c systems as it is illegal tom vent refrigerant to the atmosphere and must be contained in a sealed container of some type. An a/c machine is generally needed to properly service a system although there are guys with a vacuum pump and hose gauges that do them at home.