Sealing before insulating is going to be best, cellulose helps seal a bit when it's dense packed in walls, loose fill in attics on the other hand will not seal large gaps very well, so taking a can of foam in a can and sealing around the larger gaps in your attic and at the weak points, like where outer walls meat the ceiling along with any holes made by pipes, wires or vents should be sealed before insulating.
I agree with the air source heat pump, you don't need alot of heating in the winter and running it as A/C in the summer is most likely going to cost about the same in electricity as putting a box fan in every room in the summer.
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