Jammer, can't answer your question directly as I have no experience with a W126. But, did buy a 1996 W140 S500 a couple of years ago and can share my experience so far. Never owned a Mercedes before, so I was a "cold start".
As you probably know the W140s from 1996 to 1999 are similar. Earlier cars (92-95) are different beasts in many ways.
I do all my own maintenance on my 7 cars (some belong to the kids) and have had a number of different vehicles over the years.
I've had no major problems with my W140, just "normal" stuff like hoses, EGO sensors, etc. Biggest problem was a climate control issue that took me some time (and help) to figure out.
Mechanically, its pretty straightforward. Easier to work on than a 70s-80s Jag.
Electrically, its another story. Many of the systems are networked to each other. If you want to do your own maintenance, you've got a fair amount of reading to do. Much of the data moves around in a digital format which is cool for some purposes, but a nightmare to troubleshoot without the Mercedes troubleshooting information. Be prepared to buy a real scanner that plugs into the 38 pin connector under the hood and allows you to read much of the data stream.
Oh, and unlike some cars, you can't throw parts at the problem. Parts for these little beauties will kill you in short order.
Good news is that there is lots of help on this board.
Good luck. Sorry for the long post. Hope this helps. I'm sure others will leap in.
W140s. Folks either love 'em or hate 'em.
