I would go for a '90 or later. As for '90 being the slowest year I think that '90 thru '92 should be the same since the slow down was due to the 1st generation face lift which made the car look 100% better. As blackmercedes said, if you want the M103 engine you should get a '92. Personally I prefer the new engines such as the M104 and M119.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500