The V-8 still has the potential to require helicoiling of the aluminum block if the heads are removed. Not a problem with the iron block M103/104.
This adds considerably to the cost.
I like both engines, the M103 is a sweetie if you just want a cruiser.
The M116 and M117 V-8s are much nicer when you want some power....
I've owned lots of examples of M103/M104 and the M116/M117, and if I couldn't talk myself into the eight, then I'd want a diesel in my W126....
BTW, if you are considering a 420, do yourself a favor and look at 560SELs. They have more equipment, more power, and tended to be taken care of better by at least their 1st and maybe 2nd owners....
The 89 and up cars are the best of the breed, as well.
Jim