In the Mercedes Benz buyers guide of a few years back, their overview of the 300E was something like: "The 300E is the best buy in a used Mercedes, period, end of story".
This was several years ago, but I think this still holds true for the 124, not necessarily only the 300E. Yes, they have their acchilles heels, mostly climate control related. The valve seal problem on the M103 is not a super big deal. The headgasket problems on the M104 engine is a bigger deal. But, overall, these cars are absolutely incredible.
In response to the not being able to understand about buying a car that has some achilles heels, all I can say is, you have to drive one for awhile. You can get lucky with an American car and take it 150K if you take care of it. You might even have no problems whatsoever with an American car. But after awhile, the car will feel like a rattle trap old car. A Benz will still feel as solid as new at 200K miles and beyond.
Although my 240D is worn completely out and declared dead for now, when started, the car still drives and feels like it did when it had 17K miles on it. This is one of the biggest differences about driving a Benz.
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in