We are purchasing an historic 5,000 square foot home built in 1880 in Denver, Colorado...now 10 units. kind of a boarding house with 4 occupied units. What should we estimate as a cost to replace the 20 year old furnace and have enough money to check and rehab the valves on all the radiators and pipes. Can you include a estimate too for adding AC. all including labor. Err on the high side please. Just a ball park number will do.