My 1995 E320T is my third wagon, and still love them all.
I agree with all the previous posts about mechanical repairs. My E320T is about to reach only 30,000 miles, but have already change the radiator and the engine gasket (this last repair done under warranty). The fuses tend to give some trouble, I would just replace them all with fresh ones.
In terms of the chassis, all of my wagons have had a tendency for water leaks coming into the passenger cabin. My ex-1981 300TD had problems with the windshield gasket. By ex-1986 300TE and the E320T both had problems with the rear door gasket and also with a rear side window. Just be on the lookout for water filtration after a drive under hard rain.
Also, even though you might not use the folding rear seat often, remember to keep well lubricated all moving parts (including automatic door release latches)!