As long as we are asking home improvement questions...

I need some advice.
I recently acquired and older home. Built sometime in the 20s or 30s.
It has a good roof, the electrical was updated in the early 70s and there is some insulation in the attic. None in the walls.
Now, I am going to remodel the bathroom, it's old and needs new everything.
I would like to put spray foam in the walls and replace the old storm windows.
The problem is that they built these old houses differently back then. They have a horizontal beam between the studs that divides the wall into an upper and lower section. Can spray foam still be done?
Would there be any economical advantage to updating the windows to something more energy efficient?
Also, the house has gas space heaters and my insurance company really doesn't like those. I want to remove that and go with a Mitsubishi Mr Slim heating and cooling system and install a solar barrier in the attic.
Thoughts?