The higher you go in a house the hotter it gets, especially if it's well insulated. I'd suggest that step one should be to get 2 or 3 thermostatic extractor fans fitted to gable walls if possible. They constantly trickle air out and ramp up the extraction at a set temp. Also, if you do want AC, a mobile unit is very useful, and much less maintenance. Get builders to install a standard tumble dryer vent with an automatic air seal and then plug in a mobile AC unit as and when needed. We have this set up in main bedroom, for maybe two weeks a year, AC machine comes out of spare room, plugged in, connected to vent and cools the room a treat, when not needed, wheeled away. No huge condenser outside on the wall, no big evaporator on the bedroom wall. The one we have dropped the main bedroom temp from 31 to 20 in about an hour. It was all caused by having a bay window, curtains open, windows shut, doors shut in full sun almost all day! Over night the AC unit just hummed quietly every now and again to maintain the temp.