Getting certified is not involved. And I agree with Larry, don't convert your system to r-134a. R-12 is more efficient in your car, since they were desinged for it. Almost everyone is dissatisfied when they convert, so save yourself the headache. A/C work is one of those areas that you definitely want to fix right the first time around. The only way I would suggest converting is if you are installing all new components and converting to a parallel flow condensor - THEN you get some good results. That's what I did in the diesel, and have been very satisfied.

GregS
'84 300D, 183k
'90 300CE, 173k