I had two signs made up, one for me and the other for my mechanic.

"WARNING, THERE IS NOTHING MORE EXPENSIVE THAN A CHEAP MERCEDES"

Having said that my '87 300E (bought by my mechanic) has 220K and is running like a dream. Valve stem seals replaced at 150K, transmission resealed at about the same time. Other than that just normal maintenance.

Unfortunately my '93 300CE was the subject of the signs. It was POORLY maintained (but the price was right!)
Anziani
'93 300CE 196K
'95 E420 125K