I'm not sure if this is the right section....
Anyway, I have just started learning to model in Blender so I can put some stuff in Oblivion. Right now I'm working on a building. Currently I have about 100 vertices, and 120 faces, but I've got a fair bit to go. Have not done windows, doors or wooden framing. Have I made the shape too complex for a building? What are typical counts for things like buildings, armour/clothing, clutter items etc. for future reference?

You may want to check original objects for reference.
Just import into the Blender any similar vanilla model to the one you're making and on the top menu bar you should have summary info how many vertices and faces the imported object has (for instance such building like exterior of Anvil's lighthouse has well over 13000 vertices).

Besides, the more important thing for performance is an object's collision model than its actual visible shape. The collision should be as simple as possible without sacrificing accuracy where there's any need for that.