You might search for posts on an evaporator change, as they are similar. Also, I remember seeing this job documented with photos:very informative, on either this forum or another Mercedes forum; maybe the MB club's forum.
In any case, I read as much as I could on this because I figured I'd face it one day. You have to remoave the dash, and it sounds daunting. I learned from repairing my sunroof: it's lots worse to dread it and put it off and it wasn't nearly as bad as I expected. Great satisfaction to complete the job also. Just get information, work carefully and methodically, don't get impatient and break things; document with photos as you take it apart, keep fasteners together in some sort of order so you will have an idea where they go. It might be possible for you to look at a W126 in a salvage yard, to get an idea of what's underneath the dash pad, and if there is one near you, you can go look at it for a reference if you get in a bind.
My cousin works on the tough cases at his home shop: older vehicles which people can't afford to take to the dealer shops. He says the worst heater core/evaporator job he has ever seen is the Lincoln Navigator.
If you would like, you can document your job with photos and post here for those of us with this job in our future. And good luck!