If you want something that will offer the least amount of risk, mechanically. Go with any OM606.

Power and fuel economy is a wash, if you're good with your tools, facts and figures.

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1987 300SDL (324000)

1986 Porsche 951 (944 Turbo) (166000)

1978 Porsche 924 (99000)

1996 Nissan Pathfinder R50 (201000)