I always LOL when people immediately think the refrigerant is the problem. These Gen II 126 cars will blow frosty cold with 134a. $4.88/can vs $30-50/can. It's your call. The performance will be roughly the same due to the multi-circuit condenser and upgraded aux fans.

I see nothing said about pulling a deep vacuum on the system before charging. If that step was skipped there's water in the system and the expansion valve is frozen. What kind of flush was done, did the flushing solvent get blown out with compressed air?

Even the R12 charge was wrong. Densos have a sump, all the oil goes in the compressor on a clean system. They will circulate oil as necessary. Putting oil in the hoses is for R4's or for replacing components without flushing. Factory charge on R12 is 46.4 oz (2.9 pounds) not 42 oz. So even there he's undercharged with ridiculously high pressures.

What brand was the expansion valve? Was it the ACM/Egelhof brand and the correct size? There are 2 sizes, 18000BTU and 24000BTU, the latter is the correct size and the one stocked by most parts houses. Be aware that new expansion valves are drilled for R134a, using R12 with them can result in high head pressure and reduced system capacity (R12 has a larger molecule and different vapor pressure than R134a).

1989 should have the dual Aux fans in front of the condenser. They should be running at anything over 225PSI. If the fan clutch is dead and they aren't running, the head pressure will skyrocket.

Sometimes it's worth it realize one's own limitations and to pay a shop to do the work.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]