Thank you all for the opinions it's much appreciated. I am clear on the perils of the air suspension, but the motor is still an unknown to me. From what you all are saying it seems like a great motor, very reliable, but if it breaks you are screwed. I currently live with a Porsche 928GTS which makes me very familiar with being in that situation. Here are a few questions I still have:

Does the M189 have an impact on the car's value either good or bad?

This car being a 1967 seems to have a number of switches and controls that are different from the later 109's. For example the interior lights, air suspension raise control to name just two. Were these from the earlier W112 cars or are they specific to this car?

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1972 280SE 4.5