That car will run forever, even with major problems. (I drove my 260 for about 5 years with a blown head gasket, without realizing it.)

Mechanical/Electrical issues to be mindful of are:

OVP (already mentioned)
Voltage Regulator (already mentioned)

Most else will be fluid and rubber related, i.e.:

Radiator Leaks
PS Pump leaks
Transmission leaks
Belts and bushings
Motor Mounts

A remarkable work horse of a car.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black