The W140 is an impressive car. A different animal than any of the E's though. It has grown to have the nickname "Panzer" among several people I know... Comfort, ride, room, etc are only exceeded by the W220. It "feels" perhaps even more solid than the W220. I think it is perhaps the epitome of the MB overbuilt solidity and "old school MB-ness". (know what I mean?) Sporting is not the first thing that comes to mind. Don't get me wrong, it is VERY capable but don't expect to kick butt at a Starfest autocross. They have a few weak points. Top examples command a premium, but any of them can rack up MAJOR repair bills very easily. I would check into the status/condition of the evaporator core of any prospective purchase.
The W126 is a good choice, but I would give the nod to the W140 in just about any category. I must concede that most of the W126's I've experienced have not been collector maintained, but even if everything were perfect the W140 still has a full generation more of just about everything. The 560 will hit 60 in about 7.5 seconds and the 500 (US spec, not euro/gray market spec) closer to 9.0 secs. The 560 engine torque’s pretty well but does not have the top end of any of the new engines. The W126 500 engine is rated at 185HP/247lb-ft while the 560 was rated at 238HP/287lb-ft. The Panzer engines are rated at 322HP/354lb-ft for the DOHC 5.0, 217HP/229lb-ft for the 3.2 I6, 275HP/295lb-ft for the DOHC 4.2, and 389HP/420lb-ft for the V12. 0-60 times are quicker across the board for the W140 even though the Panzer is chubbier than the W126 by a few hundred lbs.
Bottom line is test drive 126's, test drive W140's, test drive W124 6's, test drive W124 8's...the more the better. Try to get in at least 2 examples of each model-each chassis. The treatment each car has had up till now can make a big difference when you get to the 80,000 mile and over ballpark. Don't let one bad example ruin you on a whole chassis or model. While 80K is nothing for a well maintained example, it can be the beginning of trouble for ill-cared for or just plain problematic cars. Find out which seat suits your butt... Be observant, if you didn't like the way a car handled is it because something is wrong, it is just the way the chassis is, or are the tires just worn out? Nothing beats time in the cars, and in the number of cars. Bring a notebook and put it to good use...
My personal vote is the W124. In my opinion it is the best balance or total package in the used market. Its big enough to take a group of 4 or 5 on a week long beach trip. (even the way my ex packs!) Still nimble enough to slice through traffic or around a track. There is a wide choice of engines from mild 260E's to fire-breathing 92 500E's. As plentiful as they are many parts are easy to get and (by MB standards) relatively inexpensive. DIYing is also easier than many others. The build quality is still from the days when engineers ran the show. (read: top notch!) Styling is always in the eye of the beholder, but the W124 still appears quite modern and attractive. It doesn't have the all-imposing presence of the Panzer, but still caries gobs of curb appeal. (especially if well detailed)
If you need any help finding a particular example, you know how to reach me. Drive as many as you can, when you find the right one...you'll know it. It may not be a bad idea to ask around Tech Help for recommended pre-purchase inspection tips/list. Techs and members should all be able to contribute or help. It is late and my mind is gone.