Just one full house interior. And if there are some public buildings or warehouses or something, interiors for them too; if somebody wants them made. For most of the houses, just enough interior to be visible from the outside, so that there appears to be something going on inside. Even planes with pictures stuck up against the windows would work fine.

IMO:
Putting a high poly? interior in each building is overkill.
I think it would be best to make rooms seperately that can be used as needed, instead of a whole house interior as one model.
Props are seperate also...Bare rooms and people put what props into them that they want too; same way a character is. You have the base character and add clothes, hair, etc. to it as you see fit.
Only rooms that might not be that way are the kitchen and the bathroom, because of all the built-in cabinetry and appliances. Fireplaces could be a problem too, I suppose; but didn't most every room have a fireplace to begin with in 1800s.

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