I would recommend sticking with the R12. Had to have the evaporator replaced last summer in my 1992 300E (pulled dash and all the fun that involves- for $1700). I ws out of town so my mechanic couls not reach me to see if I wanted to go the R 134 route so he kept it a R12 system).
What luck - it is in the mid 90's here in orlando and I am still freezing with the A/C to the point that I have to adjust the temp. So stick with R12 in my opinion. BTW I had him replace all the actuators and heat core as he had the dash opened up.