Don't forget the evaporator (20-30 billable hour job if IIRC
)
I wouldn't touch the W140s. If you must go newer, I'd suggest seeking out a late W124 to early W210, which are equipped with inline 6 M104 (very good motors)
A GenII W126 (especially '89-91) had some very nice upgrades from the Gen Is, and are worth looking at as well.
Here is my old one (now sold). Nice examples are able to be had relatively cheaply.
http://www.peachparts.com/shopforum/showthread.php?t=269097