Hi, again.

I understand that there are actually two different 2.8 liter engines of the late '70's and early '80's (not to be confused with a 2.8 liter M104 of the early '90's); one was a single overhead cam engine like that found in the 280SL ("pagoda" roof sports car) -- I don't know much about that engine, but it's been described as a rather simple and robust design as long as you give it frequent valve adjustments.

The other 2.8L is the one I know -- it's the double-overhead cam engine, designated M110, as found in the Gelaendewagens as well as the 280SE and 280SEL (longer wheelbase) high performance sedans, especially in european tune (i.e., without emission controls of USA models).
To recognize this engine, it's got a deep depression in the center of the valve cover, running lengthwise, and the spark plugs are situated inside this depression -- they're surrounding by the valve train on both sides.

The good side: this engine has a well-earned reputation of singing gleefully at high rpms -- it can sail along all day at 4,000 rpm or more without missing a beat (for point of reference, my GWagen is geared to do 4K at 75 mph, and I've cruised at that and higher for hours). It's sometimes described as humming like a sewing machine at ridiculously high rpm. Bottom line -- don't be afraid of revving it.

The bad side: this engine does not have hydraulic lifters -- they're solid -- and it requires periodic valve adjustment (every ~20,000 mi., or ~15,000 mi. if you really use it at high rpms a lot) -- and has a reputation for burning valves if you don't adhere to this.
A further complication is that doing a valve adjustment (a.k.a., valve lash adjustment) on this engine is particularly time consuming and frustratingly tedious. Remember, it has dual overhead cams, and the cams are on separate banks. A worn (stretched) timing chain can upset the synchrony, too.
Thus, this service is an expensive task, per "book or shop rate", and lots of owners tend to put this off to save money. So it's possible that some owners have engine problems (poor idle, etc.) because they've put off this service and now have to face the consequences of poor valve seating.

And if the worst happens (e.g., burned valve), again because of the double overhead cams and the nature of the unique "split" heads of this design (i.e., removing the head is actually like removing two heads -- twice as much work), the repairs are gut-wretchingly expensive -- literally $thousands, from a MB dealer's shop.

Bottom line -- they need proper care! They're great if they're given the right care, and can run almost forever (like a 240), but they are notoriously intolerant of neglect, starkly the opposite of a Volvo 240.

I would suspect that the two examples that you cite have been the victim of such neglect by their previous owners, who may not have realized their demanding needs.