The last house I rented was a 102-yr old house, 2 stories plus attic, 3,000 sq ft, that the landlord had central air/heat added to. (The house was in VA's Northern Neck in a village called Morattico, about 75 miles east from Richmond and 80 miles north of Norfolk.) The installer ran the air ducts through the old closets (which took up probably 50% of the room in each one...but they were fairly small anyway.) when he could. He also ran some ductwork down along some interior walls, after which a drywaller came in and enclosed them (separate contractor). In the attic were the air handlers with the filters, and the drain pipes to allow condensate to flow out. The landlord had two complete systems put in - 1 for downstairs and 1 for upstairs. The downstairs system used propane for the heating and the upstairs system used electric for the heating. The two condensor/coil units (? - I know electronics, not A/C systems - sorry!) were outside the house. After the two systems were installed, the landlord had the pulped paper-type insulation blown into the attic beneath the floorboards. The only ventilation in the attic was the vents in the window openings, one of which had an exhaust fan on it.
I didn't move into the place until after the systems were installed, so I can't really compare "before" and "after" attic temperatures. I can say that the attic didn't get extremely hot in the summer with the units running in the attic. The exhaust fan didn't kick on too often that I can recall. The attic was a little warmer in the winter, which I attributed to the heating systems and the insulation. Then again, the walls of this house had NO insulation in them and the interior surfaces were plaster, so I felt the cold wind!
The landlord told me it cost about $10,000 for both systems to be installed plus the blown-in insulation (all done by 1 company). The landlord did this to help the house sell...I moved in during July 1999 and they sold the house in September 2000. I hope this information is of use to you. Jim D/Heathsville, VA