The W210s are good cars; I don't recall seeing much difference in reliability between any of the '98+ models (I was a service advisor for a MB/BMW shop from 2005-2008; I have approximately zero experience turning wrenches but I saw a LOT of 210s-and not much else, really.
) although the E55s are, in my opinion, better than the lesser models.
Yeah, they're the same car with the same basic engine, but I didn't see valve cover gasket leaks and oil pan gasket leaks on the AMG cars as frequently as I did on the 320/430, and when I did it was usually at higher mileage. Admittedly, I didn't see as many E55s as I did the lesser models either, so that's anecdotal data that should be taken with a grain of salt, but... They're not that expensive anymore and it's a good reason to justify the extra cost over the E320/430, right?
Common issues (if my memory is serving me correctly) include:
- Crank position sensor failure; may or may not cause a check engine light, may or may not cause the engine to run poorly. Will cause stalling and no-starts at random.
- Airflow meter failures (do the throttle body gaskets @ the same time).
- Oil leaks-valve cover gaskets, small cover plate on the timing cover below the oil filter housing, sometimes (fairly rarely) upper & lower pan gaskets.
- Differential fluid leaks (the breather tube gets plugged up and they puke out of the axle seals). Also, people never change the diff fluid in these things. I'd recommend at least every 60,000 miles.
- Potential transmission issues ( @ 100k+) due to lack of maintenance (thanks to Mercedes calling it "maintenance free"). Change the fluid & filter at least every 60k. If you want to be really particular about it, halve that and treat it like the older 4G-Tronic. That may be excessive, but the fluid isn't that expensive, the filter and pan gasket are dirt cheap and I'd rather change a filter and the fluid than the whole transmission.
- Electrical connector on the front passenger side of the transmission leaks; fluid gets into the wiring harness and if left to continue for too long the fluid will wick up the harness, fill the TCU and make a big, oily mess out of it-and shorting it out in the process. Only saw it go that far twice in 3 years, but it does happen.
- Recall on the harmonic balancer on some cars, as suginami mentioned; call the dealership with the VIN number and they'll be able to tell you if it's been done or not. If it hasn't, do it ASAP.
- Occasional issues with the LCD display in the instrument cluster, which suginami also mentioned.
Of that list, the ones that I can all but guarantee you will experience are the crank position sensor, the oil leaks @ the valve cover gaskets and the cover plate, and the transmission leak @ the harness connector. The diff. fluid leak is likely if you keep the car to 150k+. The rest of them are common enough to be of note but not something that happened with every car we saw or anything like that.
I would not recommend a W211, personally. We were only just starting to see them when I left, but I saw some things that didn't impress me very much. My gut feeling is that they're not as reliable or as robust as the W210.