Here's
Mitsubishi's site. They have some pretty good advise for colder climates.
Heating | Getting More From a Ductless Mini Split in Chilly Weather: 7 Tips
The best advise would be to find a contractor who has been installing these system for awhile in colder areas.
We have a smaller home and have thought about this too. But living in an area that can get to double digits below zero kind of puts a bit of a crimp in things doesn't it.
Hope you can find your answers.