I am having a great time with my 420SEL. I think it is the greatest W126 ever made. The 560SEL has more thrust, but with the beefed up rear suspension it also has a much harsher ride. I have had no makor problems with my 170,000 mile 420SEL which I have had for 2 years. Like any car, make sure it has been well cared for, and well maintained. A regularly serviced 420SEL will be an excellent car. I am currently replacing the rear muffler assembly on my car. It is not the original MB equiptment. The second owner replaced the entire exhaust system when the car was 10 years old, and had 120,000 miles. Unfortunately for me, he saved 2-300 bucks and used an ANSA exhaust system instead of genuine MB (I'm sure OE would have gotten me 5 more years). The OE exhaust is very expensive (rear muffler is $811 Canadian) I am not going to replace it with OE, not only because the price, but because the one I'm getting has a life time warranty. Other than that, I have not had any major repairs. Things that you might look for are cruise control (goes on all older Benz models sooner or later) Climate control push buttons, etc. I thumbed through the old service records and my car did have a new timing chain and cam shafts installed. If you already have a 500SEL you'll appreciate how great these cars are. The 86 and on models look better than 85 and earlier (updated bumbers and side skirts - much nicer and larger wheels) interior also looks better. Also, don't let the fact that the 4.2L v8 is smaller than your 5l v8 fool you, there were alot of mechanical and technical advances in the w126 introduced in 86 as well. Great performer!!
Hope you find one you like.