Hi Graham,
Its all about perspective at the end of the day and how far back will it sit on the layout, in can get complex when dealing with the hieght of buildings and were they sit on the layout and to retain proportion to the reat of the layout.
Your measurements seem fine to me, its allways a good idea to make a mock up of the building first so you can get an idea of how it will look with the reat of the layout/diorama.
As for foamboard, it may be possible but once all the window apertures a cut out there won't be much left of the foamboard.
Have thought about using perspex/ heavy grade clear plastic sheet, say 100htou + and to laminate the releif of that but baring in mind you brace it on the inside, the best way would be to fit dummy floors to keep it square.
cheers
Peter