Was the conversion a proper conversion, complete with expansion valve, or did they just dump gas and oil in the system and let 'er rip?

If they changed the expansion valve, you're now stuck with 134a. The good news is that with the expansion valve changed, the Gen II W126's work well with 134a since they have a much improved condenser compared to the earlier models. It helps if you have your windows tinted. Not sure how well it works in 115˚ heat, but I've had mine running in 110˚ heat and it did just fine.

If the system has leaked out, you probably have a leak somewhere. Best to have that found and fixed before dumping gas in there. The O-ring seals in the various connections are the most prone to leaking.

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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]