The air bags are cheap. The valves I was _thinking_ are under $1000 but over $500 each to rebuild - 3 for the car plus one main one makes 4. The compressor and tank are usually fine, but not cheap if they are not fine. If the lines are bad then ????

Regarding your questions on the M189 impact on car value - I don't think it improves things because it adds anxiety. Particularly if there are other engines (3.5, 4.5, 6.3) available. On the early M189 cars the option was a smaller 6 (220 MFI) which was less power. Also, the purity and heritage of the car being original and having the "special" engine isn't that important since the car itself isn't in high demand. If it were a cabriolet then it would be a different story, and I think having the "cool" engine would be a plus. But the "cool" engine in this car would be the 6.3 not the M189.

The air suspension switched from two settings to three settings probably around the time of your car. I think the settings are Lock and Normal, and then Lock, Normal, High. It is more complicated and expensive to rebuild the EARLY valves (like my 1965 has and maybe the 1967 has). I'm not aware of other controls being different. The light switch should be the same. If it's a 6.3 it would have a tach, maybe with a 3.5. Not with a 4.5 or the 6 cylinder engines. In that case the tach goes in the hole where the clock is, and a rectangular clock (like on a fintail) goes on the level where the switches are, I think to the left of the steering wheel.

Early W108/W109 dome lights are different from later ones, but it isn't related to the 108 vs. 109. Later cars are more likely to have rear reading lights on the C pillar, but it was an option in both the SWB and LWB cars, pretty much always on the US-spec LWB cars after 1970 or so.