I love replacement windows with the nailing flange all around. These windows are easy, effecient, and a good investment.
Replacement windows of this type are ordered or bought just like the replacement door kit. Rough opening.
You make a neat rough opening with a stud or 2x4 all the way around. then the window sits in the rough opening where you now shim it up to square, once there you nail in the nailing flanges to the outer framing then mitre in your trim board and walla, your done.
Now, what you described is a bird of another color and I do not know a thing about that. Lets hear what others have to say about that.