I made a filling hose that would simply put 134a into my r12 system by hooking the two hoses together. I didn't change anything. The system held pressure for two years and cooled just OK. I bought some r12 on ebay, 2 cans and put a vacuum pump on to evacuate the 134a for about 4 hours with a heat source on the accumulator, a light bulb. I then filled the system with r12, I believe it took about 1 1/2 cans. This was about 2 years ago. My AC has worked incredible ever since. I can't believe how cold it gets. I know, I know, I didn't follow any rules, I just filled it up with 134a, then dumped that and put in R12, a real big no no, but my AC still; works, better than ever. In my opinion the 134a is way inferior to R12 in a system designed for R12. In fact I really haven't had AC in any car that uses 134A put out as good as an old R12 system. The only one that came close was a recent rental, a KIA, it had killer AC.
So, in my opinion, buy some R12 and fill it up and be happy. I have'nt tried any of the new replacement for R12, but they are just as much money as r12, so why not just use r12.
My car has the Buds Benz modern system.