it appears we need to replace one of our air conditioners. this means we also need to replace our furnace. The units to be replaced are 18yo. 120,000btu gas furnace and a 3 ton AC. We have zoned heating/cooling. we replaced the upstairs units with a 90,000 btu single stage furnace and a 13 seer single stage AC unit. we now need to replace the downstairs units. about 3300 sq feet house. what btu furnace should we get? and should we get a 2-stage or modulating furnace? What seer AC should we get and should it be a single-stage or 2-stage AC unit and how many tons? two different contractors are recommending different AC tonnage and seer, furnace btu's and one recommends a modulating furnace and the other recommends either a single stage or 2-stage furnace. we want to be cost effective and energy efficient at the same time. Additionally, we want to finish our basement. It has one above grade wall and 3 below grade walls. how do we size this equipment to also heat/cool the basement without negatively effecting the 1st floor temperature?