A 1990-91 560SEL would be the car to get.

You want all the nice updates MB did, at this point also model matters less than condition.

I like the 350SDL's as well but people are afraid of them. IMHO if its still running at this point 20+ years down the line its a good motor. They also made a handful of 350SD's which is probably one of the best combo's you can get with the SWB.

The I6 is fine but your not gaining anything, also if fuel burn is really a concern this isn't the car for you. Upkeep will eclipse fuel at this point. I'd figure on whatever you buy having to roll on a timing chain, I'd replace all the rubber under the hood along with the radiator, send the starter and alt out to be rebuilt, etc. 20+ year old stuff needs some going over for reliable service.

If I was to buy another W126 I'd buy an AMG, but for a regular one I'd hunt out a low mileage 560SEL or a 350SD if any of them still exist in good shape.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200