To remove the dash, you simply open the doors, remove the bolts on the sides (under the vinyl flaps), then remove the speaker grills and remove the bolts you find there. Then you remove the glove box, disconnect the air temp sensor and air vents. Remove the kick panels and remove those screws. And then A-pillar covers. And the cluster, of course.
(Didn't seem like that much when I did it.)
Removing the steering wheel isn't necessary, but it isn't hard and makes the job a lot easier. Just index the connection.
As for the layout underneath, this should help:
http://www.benzworld.org/forums/w123-e-ce-d-cd-td-te-class/1344511-acc-vent-vacuum-control.html
I'd lift the photos, but they're too large and look terrible when compressed further. (At least with the software I have here.)