Absolutely none of this guy's post is true. Get off the internet.

I bought a one owner 135k 2001 ML320 last fall for $3000 in Phoenix. Drove to New York, thoroughly rust proofed it with fluid film. First he belt idler went, then the Idler tensioner, then the water pump, all within a month, then the drive shaft support bearing, now the left rear lock is machine-gunning, and the bas light keeps going on and intermittently locking up a left front wheel, the doors closed more like my Chevy Corvairs, and some interior plastic parts have broken. 3 years ago I bought a 1999 E320 with over 400,000 miles, but a newer motor. It also had a few little gremlins, but it just seems like a Better Built vehicle.
I do all my own repairs, 100% of them, I even rebuilt The Idler tensioner by putting in a new General Motors alternator bearing, it was a lot of work, but I get a kick out of it. Yes, the Mercedes idler tensioner pulley uses a 1963 to mid 80s GM alternator bearing, the exact same one I put in my GM cars from the 60s and 70s and 80s. So you can't really blame that failure on anything but time. I have a spare alternator and starter ready to go just in case, but I have to figure out how to disable the BAS-ESP because it's done this crazy jam on the brakes dance several times while driving. It's very frightening, and completely unacceptable.
I'm very careful to install only made in Germany or USA or Canada parts. None of this Chinese imitation crap, I've had nothing but trouble with made-in-china water pump for my GM 6.2 diesel, and 65 Grand Prix and 65 Buick. It's unbelievable but the Chinese are imitating absolutely everything from every era. So I guess the biggest problem with this Mercedes is the BAS-ESP, LR door lock, and the doors closed really crappy. Nothing like my other two Mercedes.

2000 ML320 with 261,000. Good truck, best in snow that I have ever had. V6 is fine. Big interior, not fancy. Had to work on the prominent ailments discussed, but running strong.