Well I have blasted a still to be unnamed Benz dealer a few times on this forum, but it looks like it's time to stand up for them on this issue.
To begin with, I'm a little bit confused about your statement regarding valve guides as a known weakness. I have an '88 300E with over 165K miles, it uses NO oil. It doesn't go down on the stick AT ALL between 3,300 mile oil change intervals. Which brings me to oil change intervals. Since you purchased the car used, how could your dealer know that the oil had been changed regularly.
Benz takes care of their known problems. Case in point. In the '86 and '87 diesels, to meet California emmissions regulations, Benz installed in all their diesels, with best intentions, a trap oxidizer. It came apart with the result, in many cases, of total engine destruction.
If you were to go buy one of THOSE cars used, even with a half million miles on it, you could take it to a dealer and they will replace it with an undercar converter. And this is at a very high cost to Benz.
Now, back to hammering MB. Mercedes, as well as most every other car manufacturer, publishes a maintenance schedule that does not call for oil change intervals that are frequent enough for those who want to drive their cars for an extended period. The published intervals are good enough for the car to be in condition at trade in time, which for most people is probably no more than five or six years. If you want your valveguides and the rest of the engine to last for a long time, change your oil hot, and change your oil often.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in