The 420 SEL is a great car. Well, I think that most v-8 126 chassis benzes were great cars, but the 420 is my favorite. It has nearly all the toys of the 560 but without that stinkin self leveling rear suspension.
One thing you will notice is that the distributor cap for the 420 is more expensive than it was for your 500.
The 420 though is a good performer that also gets good mileage,(for such a large car). I see alot of them at my shop with well over 200k on them.The ones that are well maintained, still drive just like a 50k mile car.