Phil, you are new here so you probably did not see the ' AC wars' over the last couple of summers...

To get certified costs about $20 and is an OPEN book test over the internet.

R-12 is about $20 a lb...and the price is coming down... at one time an AC guy around here was charging $60 a lb....

You can get the stuff to deal with R-12 most anyplace that has AC stuff...

These Old Mercedes AC systems seem to have been designed to function well in Germany and Canada. In hot humid climates most people find they need the best ( R-12 ) refrigerant available....

If you are going to change to 134a or suggest others do ... then something else needs to be suggested to take up the natural 20% slack in the efficiency of the molecules compared to R-12.