If you want a LWB 6cyl it'll have to be a 95-up S320. The quick way to tell if you can't see the car is to look for a WDBGA33 VIN rather than a WDBGA32 VIN for the SWB model.
Get the latest year you can afford. Get a Starmark car if you can. I believe all the S320s have 5-speed transmissions while the V8s got the 5-speed in 96. The V8 fuel and ignition systems were revised in 96 or 97. Not sure about the S320.
A forum member who goes by akry has several comprehensive posts on year-to-year differences in the W140 line.
My 91 300SE was a good car but when I got the S420 I never looked back. A few repair bills might change that but only from a financial perspective. To this point I find the S420 to be well worth twice what the 300SE is worth.
I've posted what I like about the W140 over the W126 but I'll recap:
- softer ride; comparing LWB 126 to LWB 140
- better handling despite softer ride; damn roll is still there
- more detached from the road; I don't care for road feel
- smaller steering wheel, lighter steering
- softer seats and softer leather; seems to wear better too
- heat from dash vents
- when push came to shove, my 300SE couldn't keep up with an ES300, my S420 will keep up with many 'vettes
I'll leave to someone else to comment on the negatives.
83 300SD ... $1100, see cars forum