My 300E now has 225,000 miles. If mine is any guide, you are looking at a low mileage example. The only problem I've had was a corroded cylinder head that had to be removed and welded because of lack of antifreeze changing by previous owner and myself. This is definitely not the fault of the car. It would easily happen with any aluminum head engine if antifreeze is not changed.
My evaporator, the factory original is still holding and has not been recharged since 1996. It is definitely the best performing a/c of any of my cars, foreign or domestic.
I believe that the valve seals are a non issue on the '88, they had it fixed by then, if it needs them it's not a big deal. You don't have the head gasket worry that you have on the later 104 engine, so I think this is the preferred 124. Mine is definitely preferred by me because it is from the two years of production that gave us Americans a manual transmission. If I was an automatic person, I still believe I would pefer an '88 up through whenever was the end of the 103 engine.
Another plus is that you theoretically should be able to buy this car cheaper than a '90's model and IT IS BASICALLY THE SAME CAR.
Tell your wife that these cars are differentiated by series, not year model. This means that an '86 to '95 are virtually the same, why pay extra money simply because it was put together more recently? Miles would be more important to me than year.