i'm making in this period a line of resin buildings for the beginning of the century (actually rthey can works from 1890 to 1945 or more), and i am using plaster to start.
if you don't have time, you can use for example thin layer of extruded ***styrene, dunno which is the best.
i seriously suggest you to make moulds of windows frames and door (or buy them!), because those are the most important parts in buildings. it doesn't matter if the wall is too rough or too flat, if you have nice windows instead of holes carved or cutted from the foam everything will be great!!!
good luck!!