Unless there is a 143, I think you mean 134A. I can't recommend you convert. I, too, have used Freeze12 in a r12 system. Worked well, but I can't say how it compared to original R12 as it was my sons college car.
R12 is allegedly still available, albeit expensive (in the states), but I do not know the legality of buying/selling/using.
My 560 has been converted (by PO), and it is marginal in the 100F plus, 70% humidity Alabama Summer. I suspect it does not cool as well as R12 from what I have read. Biggest Problem I see is the evaporator is tiny, and the condenser is not much larger. All this coupled with a weak blower (cfm) and marginal vent opening cross-sectional area. Just not enough BTU's, I think.
BenzWorld.org forum is one of the largest Mercedes-Benz owner websites offering the most comprehensive collection of Mercedes-Benz information anywhere in the world. The site includes MB Forums, News, Galleries, Publications, Classifieds, Events and much more!