Use R12 - its not that expensive - visit http://store.lenzdist.com/ and call them to see what their price is.

Here is a decent table of pressures/ambient temps

R134a and R12 High and Low Pressure, Ambient and Vent (Evaporator) Temp

EDIT: According to ALLdata the 1983 380SEC takes 2.866 lbs (45.85 oz or 1300 grams) of R12

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE