I would tend to stay away from the 350 diesels. Now, those models may have the best refinements, but for piece of mind mechanically, I'd go for a 300SDL. Although, you could always swap an engine from a 300SDL into a 350SD/L, I would love that.
-Joe

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.