Problem with converting BSP to mesh is you will need to add collision (blocking volumes). Problem with BSP buildings is that when ever you use the subtract brush it may lead to invisble collision shelves not visible in the editor, and if you add trim (window and door frames) there are extra polys, albeit few.
Plus side of meshes you can vertex paint them. Plus side of BSP it has its own collision.
Making a building inside of the editor, build to the grid (vertices on the grid points). After layering up your walls with addative brushes and applying textures, copy-n-paste the entire structure elsewhere and do whatever rotation you want to the group. Vertex editing will allow you to make destroyed shapes.
Alternatively, make a builder brush that encloses your new building and deintersect, move the resulting red brush away and add. It will preserve your textures and make it into a single brush (this makes the brush a single draw call for performance reasons, each texture/material used adds a draw call). Remember to keep it simple. Floors and ceilings should be seperate brushes to prevent light bleed in corners, and wall vertices should terminate at those intersection lines because polys are lit per vertex.
Much simpler to make buildings if you have a modelling program, and you can integrate collision volumes as part of the import. Blender is a good free 3d program to play with.
I made lots of meshes for Yakovlevo_CA, finally figured it out with the "Control Tower", which was the last to be added.
I know, TMI....