If you are out to save money you may want to visit your local Home Improvemnt Contractor.
Bring a set of plans (or a crude drawing) of what you have now.
Even the best Home Improvement Contractors have special order windows in the warehouse that, for one reason or the other, they are stuck with. Good windows!
An example of why he have them is that his customer may have lost his job after the order was placed and it may have been to late to stop the order from the factory.
You have a screen porch that you will have to frame for the new windows. If you can find windows within a size you can work with, The H.I.C. will usually go out of his/her way to help you out. This means you will probably get the windows at, or near wholesale!
If you need to have a window with a wood interior, I would look for the brand names, "Pella" or "Anderson".
If you are looking for vinyl windows, check in your area to find out what the old- time contractors use.
I would go to Mega home building materials store, (if needed), only after I shopped the little guy!