e420A1
I get my information from www.alldatadiy.com. It is a paid subscription. Minuses : I find it hard to understand the way the site is structured, they have a "search" function which hardly ever returns what you are looking for and a lot of the illustrations are miniscule. (you can get around this if you copy them and then blow them up using any picture editing software) On the plus side, it is reatively cheap ($25 per vehicle for one year and $15 to renew for subsequent years or to add another vehicle) and it does have a lot of information. I have never used the MB CD's so I can't compare to those.
Regarding your question about how it ought to feel. Based on my '97 420 and an '00 320 that I drive occasionally I would say the transmissions are smooth but not extremely smooth. You can certainly feel them shift.

There is a host of reasons why the transmission can go into limp mode, some are severe, some are benign, don't despair yet.