An 85 300D would be better for maintenance & parts hands down. Parts for the W123s are so much cheaper than for the W124s, also the W124s have known to break parts shortly after replacing them in cycles where as the W123 typically is a fix every 25years or so.
Most people looking into the mid 90s 300Ds wouldn't be very appealed to the mid 80s 300Ds as they don't have "as much power." Yes they have 125hp but I believe they are a bit heavier, they also get upper 20s for average all around mileage versus the 30s the 87+ would get (because of aerodynamic drag and gearing), and they don't have all the nifty features (things to break) like the newer ones do.