Our house is new(ish) -- about 12 years old. It was built with the intent to add AC -- the 40A circuit, and breakout box are already in place. The duct work should be good. It's wrapped in insulation, at least, which I'm not going to undo. And the air return is at the top of the stairs.

I'm not currently looking at heat pumps, though I've heard good things about them. That will be on the agenda when we have to do a total system replacement. Right now, we're just looking for the cooling (which we currently really only need for maybe one month of the year, climate change not withstanding).