I appreciate the second opinion regarding the R134. I have a 91 300E with non-working air. I've been given opinions both ways as far as how to approach restoring it.
My daughter's 190E (bought last November) was converted to R134 by the PO.
They told us it just needed to be re-charged, which translated to me that there's a leak somewhere. I'll find out the virtues, or not, of a converted system this summer when I have to deal with it.

I'm sticking with my head gasket opinion though. I've seen a ton of them and read about 10 times more on the forums. My 300E is on it's second new head gasket (first one done at a dealership, second by me). I look at most every E-class I see for sale and without exaggeration half of them either are near un-drivable or show the classic signs. A few blatantly bad ones claimed not to know it. One of them had globs of brown sludge in the coolant tank and said he wasn't aware of it. I don't see how, the cooling system was completely fouled. Another guy, car almost as bad as the former, actually got mad at me when I pointed it out saying I was just trying to get a lower price.