I wouldn't use balsa for any wood. It has a large grain and just doesn't work that well. A model railroader can give you more reasons than I. Basswood is the favored real wood for diorama use. It has a much finer grain and is available for hobby use in many sizes.
For windows and doors, look in the model railroad aisle (web sites) for O scale units. A number of companies make ready to paint and install units, usually in plastic.
Sorry I'm not helpful with company names, but model railroad sites will get you all the info you need.