I see it much the same as the generic building manager in the old ASD (Airport and Scenery Designer):
1. Allow selection of building type (# of sides, etc.) and roof type (peaked, etc.).
2. Allow selection of length, width, and height (height of each of the three sections of a building - first floor, middle floors, upper floor). Display the sides of the building as you will see it in FS (double display if needed, for length and width), as you are editing.
3. Allow selection of the three different textures for a building (first floor, middle floors, upper floor) and their "stretch" across the building.
4. Allow selection of the number of middle floors. Displayed building can change in height if desired, but not required.
5. Allow selection of the roof texture, with roof display (third display). Allow tiling, if possible.
In ASD this was a 2D display (three displays), but can be a 3D display if you want to.
FSDeveloper Wiki PDF about what is possible, complete with thumbnail pictures in the Appendix:
http://www.fsdeveloper.com/wiki/index.php?title=Generic_Buildings
Hope this helps,