If the '87 ran slower than your '84, then there is something wrong with the '87. When my wife first drove our '87 TD after drving the '83, the first 3 words she said were "oh my God!". As speedy said, these cars haul, especially on the highway. You just nudge the petal a little and within 2-3 seconds you're gone. Passing someone on the highway isn't even an issue. In my '83 if I want to pass someone, I have to do more anticipating(planning), not so with the '87.
With buying another car, you're sort of starting all over again, getting familar with the car, sorting it out, etc. Where as you already know your '84. Several people here said that once you drive a properly running W124 diesel, you never go back to a W123, and they are right. However, I do like the interior of the W123 better. I like the chrome trim which the W124 is completely devoid of, except for the door handles.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member