Peter,
Some of the mid '70s 280 twin-cam engines had thermal reactors (a sort of converter) bolted to the head, in place of the exhaust manifold. They kind of look like a pair of large coffee cans. The heat they generate may cause damage to the head over time. The reactors may crack and leak exhaust and good replacements may be hard to find. Getting
rid of the reactors involves finding a conventional exhaust manifold and matching exhaust system parts for replacement. I think the 280 after '75 had the thermal reactors.

Some '68 and '69 carbureted sixes had smog pumps.
'70 to 72 250 just used a series of temperature and RPM regulated vacuum valves that controlled ignition timing to reduce emmissions.
I think the 280 4V Solexes are a bigger headache than the twin 2V Zeniths on the 250. The Weber replacement I've seen for the Solex involves replacing the 4V wth a 2V and adapter.
'72 and later cars had lowered compression to
used unleaded regular gas. Earlier, higher compression engines may have slightly better performance and fuel mileage. Condition of the automatic trans has a big effect here too.
Some later Mercedes automatics used a torque converter instead of a fluid coupling. I Don't know if or when this applied to the 280.

I mentioned cold starting diesels because a well worn diesel with low compression may be impossible to start cold, even on a warm day.
If you go to look at one thats already been warmed up, it may start up just fine until it cools off. A friend had a high mileage 240D that eventually would only crank start in warm weather and as it got worse then would only push start. Finally it had to be push started even after the engine warmed up!

Happy Hunting, Mark

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DrDKW