Money to burn? Buy an early W140. Want something cheaper to run, but not as luxurious? Buy the W126.
The best W126, in my opinion is either the 300SE/SEL or the 560SEL. The 300 is a superb car for it's relative economy of operation. The M103 is super reliable, easy to work on, and pretty frugal on the fuel for such a large car. The 560 is priced the same as the 420 in the used market, so go for the extra power and M117 (M116 has a few more issues) engine.
That said, I wouldn't pass up a super cherry 420.
For the W140, the 92-93 cars will give you many headaches, but are wonderful big cars to drive. The list is long of things that can go wrong thanks to the W140 being a huge leap in technology contained on a rolling chassis. In 1992, there was no car on the planet more complex. Do some searches for specific problems.
1998 C230 "Black Betty"