I buy a lot of parts from Jon at the classic center. Their prices are pretty reasonable for OE parts in my opinion. If you're part of the car club you get 10% off on parts from the classic center and dealers.

I have a 1991 300SE.
I usually pull 22-24 mpg highway at 70 mph.
City usually 17
It's slow, a/c compressor kick on is vey noticeable.
I've never driven a 420 or 560 so I can't compare.
Head was rebuilt 30k miles ago, doesn't burn any oil but the head gasket is leaking externally back by cylinder 6. The next headgasket will be #4 since this car was new.
It screams on the interstate, but the engine loves it.

I would definitely buy another 300se, my next w126 I'll purchase based off condition of the car and not so much engine.

__________________
"It's what non-car people don't get. They see all cars as just ton-and-a-half, two tons of wires, glass, metal and rubber. That's all they see. People like you or I know, we have an unshakable belief that cars are living entities..... You can develop a relationship with a car, and that’s just what non-car people don't get."
-Jeremy Clarkson