ok, Blender does NOT handle complex modeling well from my experience. If you want to do an entire building in high-detail, you will feel the strain.
Blender’s rendering limit is about 2million verts and 256 lights, and in my experience this is quite easy to exceed. Now if you are not beveling your surfaces and not going for photorealistic lighting, than it should have no problem.
Blender’s file system can handle large amounts of data quite well (although the amount of time to save a load a file will climb as you add more detail), and if you break your building up into layers you should be able to circumvent the rendering limits.
For walkthroughs, there is no dynamic face culling (i.e. removing faces from the calculation that you can’t see and would obviosly not be in the frame) built into the game engine, so you will have to keep your models relativly low in detail and the lighting relativly simple.
I hope this helps!