An S320 wouldn't be a bad choice at all. I know a guy who owns one and even with a lead foot he averages 20 mpg. Not bad at all for such a heavy car. Just be aware of the fact that everything related to the W140 chassis is much more expensive than you're used to, with your W126. I truly can't wait to be rolling around in a W140, regardless of the engine under the hood and the options included in the vehicle assembly. I just don't want to get one that will be a money pit, and in order to avoid a money pit, I'd probably have to pay significantly up front at the purchase. Not quite ready for that yet. I just have to be patient.

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1999 Mercedes-Benz S600, 103K miles - garage queen
1988 Mercedes-Benz 560SEL, 89K miles - daily driver
2007 Hyundai Sonata Limited, 31K - daily driver