Believe you will enjoy your 420SEL. I have 240k on mine and still going strong. Expect to need a valve job at 200k plus a catalytic converter. If you start to use a lot of oil, go ahead and get the valve job - will run like a new car.
With as many miles as yours has, suspect timing chain has already been replaced, but no guarantees - can't hurt to check.
Only other problem with W126 is a/c problems - the Achilles' Heal of M-B. If you're going to have a big dollar expense some day, odds are it will be a/c system. Good news is once problems are isolated (monovalve, aux water pump, pushbutton unit), system will work again quite well. Others report power window and sunroof problems, but so far I haven't had any.
Believe 420 SEL is one of the star performers of the W126 class - decent mileage and great acceleration, without rear suspension problems of 560 SEL/SEC. Also believe W126 is one of best looking M-B...classic lines.