Well, I completed the floor layout for the first floor in about an hour in sketchup. It was actually quite simple once I made the basic measurements of each room and just estimated 5 1/2" thickness for each wall (4" old-school studs and 3/4" each side for lathe and plaster).
From this alone I can determine that the ground floor has 1220 sq. ft. of conditioned space, and can estimate that the upstairs is about 1050, giving me a total of 2270 sq. ft. of conditioned space in the house, less than I had thought.
Next I'll work on the upstairs and possibly basement, then see if I can overlay electrical, plumbing, and HVAC on the layout. So far so good!