I am very happy with my 1986 420SEL. As far as I know there weren't too many changes between 86 and 91, but it would be safe to assume that the later the model the more likely it would be to have these changes. I don't know what year but I think passenger side air bags were introduced later in the series. My 86 only has a driver side air bag. I have gotten respectable gas mileage with my 420 (for a car of its size and the way I drive it. I average 17 mpg when I split city and hiway and I have gotten as high as 23 mpg from three straight tanks on a very long hiway road trip. I think you would be satisfied with the power it has. The 560 is more powerful and has some more cool features like adjustable rear seats etc., but eats more gas and the rear suspension is beefed up a little more making the ride a bit harsher.
I would say to get the latest model that meets your budget for this car. Check it our thoroughly. The biggest things I have had to replace/repair were the exhaust system centre resonator and rear muffler assmebly - very expensive, the secondary air pump and the sunroof. I think there was a conversion for the sunroof as early models had lots of problems. They say if the conversion wasn;t done you could expect to repair the sunroof every few years. Check to see what year they came out with the updated sunroof.
Great cars, I'm sure you'll yours when you get it.
1995 E420 - retired
1986 420SEL - retired