There is a reason why they are a bargain. AMG or not they still have all the problems associated with that generation of Mercedes. All 3 of them are from time Mercedes quality was in the sewer. This is a hated time in Mercedes history for pretty much all Mercedes fans.
I personally know somebody who had to basically throw a W210 away because rust took it over and repairing it would be too costly. If it was a V8 he could at least sell the engine. But as a low powered diesel he actually had to pay the scrap yard to come pick it up.
It's not only at the obvious and easy to look places like wings that they rust. Take the bumpers and any plastic body work off and be horrified with the tin worm chewing away at your precious Benz.
Engine wise they are fine. But this is not enough. Rust and all sorts of structural problems and also some electrical makes them not for the faint-hearted.
With Mercedes goes basically like this, if you can't afford a W212, W222, W204 etc and still want to drive a Mercedes you better go back to the W124, W126 etc.
Sure there are a few who will say they have a W210 and are trouble free. But they are the exception to the rule. Some even consider addressing rust often being as being trouble free. They will say, just take care of the rust as it pops. Yes, right, like every weekend. Now seriously.Unless you are very adventurous, a W210 is to be avoided. Doesn't talk much reading or talking to ex owners to realize that.