Dash removals are some of the most arduous repairs done on our cars. Here's the FSM article on the subject:
Section 83.100 - removal of the heater box.
https://www.startekinfo.com/StarTek/outside/11883/?requestedDocId=11883
I fear this may one day be in my future as well.
As for the cracked dash, if I recall correctly there isn't a lot that can be done to fix the problem without it looking funny. I have a plastic dash cover hiding an other good looking dash that is cracked to kingdom come.
It's part of the trade off we have for the rust free metal gods. An uncracked dash with lots of rust or a cracked dash with no rust.
Edit: I take back what I said. Here's a great read on a guy who "flocked" his dash on a 300D with very good results.
http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1536315-cracked-dashboard-fix.html