I've had the same problem... there isn't much info out there on the W140 service procedures. Can't speak to previous MB CD's because I've not actually worked with one.
I find this forum to be a tremendous help... lots of knowledge here. Also, have found the ALLDATA subscription service to be very good... although it requires some familiarity with the car and isn't particularly "user friendly" for navigating around. When I first started using it, I was pretty dissatisfied, but its helped me on a number of occasions... so, I'd have to rate it "very good" overall.
Probably the two most important things to me have been electrical schematics and part locator functions. The way stuff is spread all over the car, you won't be able to find many problems without them... you can find both of these at ALLDATA.
I think MB offers an on-line service website, but its pretty expensive.
If anybody sees the MB W140 service CD come out, please post and let us know its out.
Hope this helps, KenP