Paint.net is a wonderful program that I use every day. However, I don't believe it will handle the layered PSD files. GIMP is a free program, that I have 0 experience with, but several people have successfully used to make better repaints than mine
. I'm purely using photoshop for the paint kits. I know there are GIMP tutorials out there.
Are you trying to make one plane, or do several? I don't want to discourage you from learning, the more people painting the better. But, if all you want is your name on it, or a different set of squadron codes, I can help you get it done, or send you something.