You didn't specify the model year of your friend's 300D and
you've confused a type of automotive freon - R-134a - with the Harrison R-4 compressor that was used with the W123 equipped with the late style automatic climate control. Early W123 300Ds were equipped with York A/C compressors.
Unless you have auto A/C experience, this may be a job best left to professionals. Specialized tools are required (including a vacuum pump) and there are several decisions you'll have to make which will ultimately impact the A/C system's efficiency and longevity:
* Reason for compressor renewal? If the existing compressor seized, then you should replace the suction manifold that attaches to the compressor, in lieu of flushing. In the mid-'80s M-B released Service Information sheets indicating that hose flushing did not adequately remove metal particles from the hoses; unflushed particles would subsequently cause failures in renewed compressors.
* Compressor - new or rebuilt ? - new R-4 compressors are not particularly expensive and are far more reliable than rebuilt units. It's false economy to install anything but a new unit.
* Refrigerant - your freon choices are R-12 and R-134a; R-12, which requires a license to purchase, costs more than $35/lb; R-134a, less than $8. The maximum freon charge for an R-4 equipped 300D is approx 2.6 lbs of R-12, approx. 2.08 lbs for R-134. R-134a operates at a higher pressure and generally doesn't cool as well as R-12; higher operating pressures may precipitate failures in fatigued refrigerant hoses. Operating environment should be taken into consideration if you are looking for maximum cooling; in hotter climes the nod for cooling efficiency still goes to R-12. If you install R-134a, you may wish to consider installing a new R-134a calibrated expansion valve to achieve maximum cooling. Converting to R-134a will require installation of different Schrader valve fittings and the use of specific "O" rings to minimize leaks at charge valves and hose couplings.
* Refrigerant oil - R-134a requires a different oil (PAG) than R-12 (mineral oil) but apparently complete removal of existing mineral oil is not as critical as it was once thought to be. Either way, you must ensure that the correct volume of oil is present.
* Receiver/drier - should be replaced on principle, regardless of refrigerant choice.
* Condenser - does not require renewal during a R-134a conversion. But should be thoroughly flushed to remove existing oil and debris.