Hi, Mike. I can give you some good free advise here. Buy the windows! Even replacement windows using single pane glass will require a big investment in jigs, fixtures, router bits and so on. And then, unless you are already a very experienced woodworker with a complete shop, the windows you make will not be as good as what you can buy.
To get the optimum efficiency from your windows, you should be using double pane, sealed panes with argon in the sealed space and low-E coating on the glass.
If you are building a cabin it the woods or something, make friends with a person from a window replacement company and ask them to save you some good old windows that they have removed from another house. If you are building a children's playhouse, check out Norm's plans for that as he had basic windows in that.