I like the Haynes manual for some of the pictures... but it leaves some things out totally...
So you also need the Factory shop manuals... and I really prefer the paper ones.... random access is faster... LOL
Here is what I have picked up so far....

1. 123 chassis manual vol 1
2. 123 chassis manual vol 2
3. 616 engine manual
4. 123 heating,ac and climate control manual
5. 123 electrical manual ( I think it has the vacuum routing also)
6. 123 manual trans overhaul manual
7. maintenance manual 123

8. engine manual 617.952 turbo
9. engine manual 110 gas

The official MB manuals are not very easy to read... but they have all sorts of info not in the Haynes manual.