First let me say I agree with the egress idea. Its important for a fireman and gear to be able to get inside. Egress requirements vary from place to place. Some locales only require them if there is a bedroom in the basement.
As far as you getting out - I know for a fact that a large teen (over 6' tall, easily weighing 250 lbs) can get out and in a window that size. You and your wife may not be as agile or may have more girth.
To frame it, build a frame in your wall parallel to the window well. Then lay in plywood or 1x lumber to create the deep-set window. Its as if you are building a sit-in window. I used 1/4 inch luaun plywood.