My favorite two series W124s with 6-cylinders were the early 3.0L M103s (86-88) and 2.8L & 3.2L M104s (93-95). The M104 has a lot more crap to deal with, as it is a more complex engine. M103 is simpler. As long as you get an 86-88 300E (the ones with perfect axle and gearing), you won't miss the extra power.
1988 Mercedes-Benz 260E