To have many mesh and each one with 6 materials, I used the same cube, and duplicated it many times, so I had only 6 materials and I could select and texture the single faces of the whole building in world.
I saw that if I add more mesh instead of duplicate the same one, I'd have 6-12-18 materials and so on.
I imported the spiral staircase as single item, not in the same .dae with the building.
In the next building, a bigger industrial one, I'll try to join some part to have midsize blocks and to UV map these blocks in Blender.
Bye the way I have plenty of time to test mesh because at the moment less then 5-10% of resident have the right viewer for Mesh. I can't ruine my sim wich is in the official destination guide by rezzing mesh seen like sphere or bad textured prim from 90% of the visitors. I rez Mesh only in hidden sandboxes. And I think the same, at the moment, about selling mesh items. I hope things will change in a hurry.