By all means have a PPI done by a MB-trained tech who will be able to spot any imminent expensive repairs. I suspect you realize that the 8-cylinder engines are more expensive to maintain and fuel than the 6-cylinder ones. If you're a DIY'er the 6-cylinder engines are simpler to work on. Check the Kelley Blue Book value (kbb.com) for a 1992 400E in your area.

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Fred Hoelzle