Some thoughts on this job.
The first car done, the 300TD, took a while to do. I did a lot and was a little lazy doing the job. The second car, the 240D took 3 days to do. We took lots of pictures and check everything over. I also spent a couple of hours re-gluing the AC control panel unit. I had ordered one and I eventually got it, but my Daughter needed to get back to school. I have it and will probably never install it
I probably could have gotten the job done in one long day, but I don't like to work that way.
While your there. Consider replacing the following
All vacuum elements.
All vacuum connections
Of course the evaporator
Possibly the heater core ( this could be fixed with leak stop, I would not do this job JUST for a leaking heater core.)
All light bulbs
Check all the plastic vacuum tubes for damage.
Check the condition of the evaporator box and epoxy any weak areas.
Air temp foam tube, consider replacing it with a hard plastic tube
Evaporator box drain tube. I used a garbage disposal to sink drain adapter tube. ( make sure you install your tube before installing the evap box in the car )
Rebuild the instrument cluster, its out
Check all the vents in the dash for broken tabs where they mount. They use 4 tabs integrated in the vent to hold it in position, these break
Time to really clean up the mess under dash and center console
Here is the post I did that is just a word description of the evaporator installation. It has some lengthly considerations posted. This post along with the one below should be adequate for anyone doing this job.
Order for installing evap box in a W123.
Dave