I have had the M103 inline-6, and currently have the M104 DOHC 6-cylinder and the M119 DOHC V8 in two different W124 chassis (wagon & sedan).
While the 6-cylinder cars are IMHO better balanced, and lower to maintain, there is definitely no substitute for cubic-inches.
The inline-6 cylinder engines need to be revved to really get them in the powerband.
Since you don't have to rev the M119 V8 to get a healthy amount of torque, you can achieve the same gas mileage with the 400E (almost never with our 500E).
Changing motor-mounts on a M119 W124 is an exercise in patience and perserverence. I've done the M103 and M104, but will NOT tackle the M119 myself.
If you HAVE to get a car now, get the one with the best maint. history, otherwise wait to find that "cream-puff" of a 400E/E420 with impeccable records.
Hope that helps,
1988 360TE AMG