Stay away from engines that have been abused by hard use and lack of maintenance. Oil/filter changes especially are important in an engine's health. The engines that see the most traffic on this forum are the OM60x series and the OM61x series. For these engines you will have the best forum support, which can be critical to a new owner. Engines older than these are still good but parts may be harder to get and there will be less support. Newer engines are better in terms of smooth, quiet operation, more power, and better emissions control, but will be more electronic, possibly harder to diagnose, more expensive to repair or get repaired, and more expensive to purchase.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970