I have a 1986 420SEL, live in Texas, have had it for 3 years. 100K miles. AC has never had Freon 12 added and it cools very good. Only complaint is the fan in auto mode blows very forcefully and loudly.
So have your checked for leaks. I would doubt a 134a conversion would be very satisfactory. No one I know in Texas who has done a conversion has been satisfied.