With Mexico it all depends on where you live and who you know...
My wife and I have had excellent medical care, but this is in Mexico City, where the doctors are mostly trained in the US. In the small towns, you are on your own. I agree with the dental work - it is much better in the US.
I too will be returning in winter to Morelos for a couple of weeks. Perfect weather.
I also would be leary of driving an MB. In much of Mexico it is a huge status symbol, meaning you are wealthy and it can make you a target of all kinds of problems. And it doesn't really matter how old it is!
The service at car dealers is usually very good. I had a Jetta and a Chrysler and I would say that the service departments were excellent (in Mexico City and Cuernavaca), courteous, always solved my problems, and were not too expensive. Remember that wealthy Mexicans drive nice cars, want them and can afford to have them well maintained.
I have a friend who has an ML320 and while she has had many of the usual electrical problems with it she says the dealer is very good.
Buena suerte, amigo.

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John Gillespie
1988 560sec - 192k miles
2006 CLK500 - 40K miles
1995 E300 - 202k miles