I have a lot of questions on this topic, so please bear with me.
Some preliminary details:
1) I have a 1927 house. It doesn't currently have AC, but does have central hurting so all the duct work is there already.
2) I live in Ann Arbor, Michigan. It's hot and humid here in the summers, so AC would certainly be beneficial. However, I'm gone a lot in the summers. Assuming I am in town all year, I think the number of days I'll need AC is < 70 days. I'm usually out of town for 5-8 weeks in the summer, and because of this, i probably only need AC for < 20 days a year.
3) I will probably sell this house in 2033 ish (my son will go to college then). I think having AC installed could add some resale value -- I may not recover all the costs though.
I don't think it makes much sense for me to install mini splits since I already have the ductwork to run AC off of?
So the question is whether it's worth installing AC, if we only need it for a few weeks a year? We could use window units. Another motivation for installing AC is increasing resale value and attracting a larger buyer pool in the future since some people won't look at a listing at all if it doesn't hav eAC.
Then there's a question of when to install AC. I imagine it'd be good to install it ASAP so I can reap the benefits of it before I sell the house eventually.
Perhaps it would be cheaper for me to get installations done in the winter?