Modeling over photo backgrounds is certainly a viable approach. If you want to model specific buildings for which you have photos/drawings, there are some things to consider.
For example, most building photos you'll find are taken at an angle, which will not really work as a background for ortho windows. When I have such a photo, I'll generally come up with my own external blueprints for it - I do this by assuming certain measurements, like that the front door is x meters tall by x meters wide, and based on that I can estimate the size of the exterior wall surrounding the door. While not 100% accurate, this can produce good results.
Modeling the interiors "inside" the exterior can be tricky --- I'd almost recommend doing the interior first if you can. I'd love to see layers in AC3D, which would help the modeling of interiors/exteriors immensely. In the only building I've modeled, I modeled the outside first, then copied the building to a new file, placing a bounding box over where the interior walls and doors/windows would go. I then removed the parts I didn't need and modeled the interior as a separate model. This way the exterior walls didn't get in the way. Once finished, I merged the interior/exterior together. You could also just hide the exterior polygons, but I use Hide/Unhide a lot, and it would have been a pain to hide interior walls and then unhide them (thereby unhiding all of the exteriors as well) --- this is why I opted for the new file.
Hope that makes sense.
When choosing between polys, primitives, lines, etc, it will all depend on what you're going to do with the final product. If you want these buildings to actually be loaded into a game environment, then the answer lies in how the game environment needs to see the data. For many formats a "triangle soup" (a bunch of triangles that make up your model) is fine, which means you can use any combination of polys/primitives/etc., but if your engine uses/needs BSP/CSG, you'll want to stick to closed primitives.
Rubble, holes, and debris are not as easy. The CSG plugin would work wonders for making blast holes, but can often produce a *lot* of triangles - this is normally only an issue if you're loading the data directly into a game engine, though.
For rubble, it may seem silly, but getting photos of rocks/planks/etc can be handy for creating random rubble. Modeling a rock seems trivial enough to do without a model, but for most people (myself included) more realistic results are achieved when you have a real rock to work with.
Also, don't feel like you have to model every random piece of rubble - you can get by with surprisingly few rubble models, and by stretching/rotating/resizing them, you can copy the same objects many times over and create a decent looking "random pile".
These are all just opinions, of course, but I hope they help.
Dennis
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