I had 420sel and now I have 500sel. Both cars I fix myself. Major difference in fixing them is parts price. w140 is more expensive to maintane and salvage yards prices are still high. And as somebody already said w140 has way more electronics than w126. Drivability wise they both feel very solid cars, very roomy inside ans very smooth. 500sel feels faster and more quite. A lot of details were added like vacuum windows,door assist,trunk handle,parking antennas,adjustable seat bottom,foldable mirrors,nicer sound system,split climate control. Quality wise I can say 1 thing-it's just luck. Somebody buys new car and most of cars life it spends at dealer. I bought my 500sel that looked to be not very well maintained, all messed up,non drivable, engine looked overheated, chain skipped 2 teeth. I started the original engine and drove the car and I was amazed that it still runs and drives very nice even after all damages made to it. So I can state that m119 is very dependable mechanical wise. I love w140. You will be very happy with it.
Good luck
Victor

__________________
1999 ML430 Silver 135k ml
Smoked LED taillights, Alpine in-dash DVD player, 12" sub, Pioneer components, 2 amps, 4200k HIDs, ML55 headlights, 20" rims, mirror LED blinkers

SOLD. 1996 E320 145k ml. Burgundy

SOLD but miss it everyday 1992 500sel Black
Chime buzzer mod,Alpine CD/MP3 player with stock amp,Euro headlights,S600 grill,20" rims, BEST CAR EVER!

SOLD. 1990 300E White
187k ml,smoked tails,Euro headlights,17rims,Intake,12" sub in first aid kit,alarm/autostart,buzzer mod