After a long debate and considering all the pros and cons (it took about a minute and a half), we decided to try "The Great Stuff" to make the basic mountain shapes.
We still may use plaster cloth or styrofoam, but we wanted to start with "Great Stuff".
We used the one that is minimum expendable.

At a later point, when somebody asks how we build this layout and what kind of materials we used, we can always say: "We used a lot of great stuff to build this layout"