If it isn't exhibiting any rod-bender, or head gasket issues (which are the Achilles heel for this car) the price is fair for a good running SDL.

But you mentioned it had some rust, so get underneath VERY well and check around the jackpoints, floorpans, trunk, under rear quarter panels, and around all wheelwells

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1987 300SDL #1, 1987 300SDL #2, 1980 240D, 1982 300SD, 1994 S350, 1990 350SDL, 1991 350SD, 1985 300D, 2005 E320CDI

Gone, but not forgotten: 1981 300SD, 1982 300SD, 1987 300SDL, 1983 300TD, 1980 300CD, 1981 300SD #2, 1987 300D, 1987 300TDT, 1980 300D