Most R4 compressors fail because of incorrect oil and amount used. Then, there's the crap shoot rebuilt R4s that may work 4 seasons.

The bottle neck in cooling capacity is the condenser. In 1985 the 300SD got an up graded condenser which makes ice. My 85 SD with it's R$ and R134 is a meat locker.

btw, there is an aftermarket P-flow for the W126 that is a direct bolt in replacement. I'm running one in the '84 SD with a sanden pumping R152a, aka dust off, and it cools fine in our 100f Texas summers. I can't even feel the compressor kick on.