The difference between new construction doors/windows and replacement doors/windows is fairly simple. New construction doors/windows have a mounting flange that rests on the wall sheathing(plywood/OSB/etc.) The doors/windows are installed before the final wall treatment(siding). Using new construction doors/windows on a house with siding in place means removing/cutting the siding to allow the flange to rest on the sheathing. Replacement doors/windows do not have that flange, allowing for installation without disturbing the siding. Some trimming is necessary with either type.

JaneSylvia, I'm no window/door expert but my DH and I have some experience. I'm only familiar with Argon gas in Andersen windows. Its heavier than air and has a higher R-value than regular air. Our first dual pane windows were made in 1975 by International (no longer in business). The seals broke on them within 10 years and were so bad you could hardly see through them. Whatever you do, make sure they have at least a 20-yr guarantee against glass failure. You absolutely want LOW-E glass. Having had both regular dual pane and Low-E (Andersen casement) there is no comparison between the two. Plain dual-pane doesn't do much for keeping the heat out in summer. My DH replaced windows and 2 patio doors in a rental house with Jeldwen from Home Depot. Retro is much easier if you're doing the install, but it does reduce the visible glass so it makes your window smaller. Of course, its only as secure as your original frame when the house was built. The Jeldwen from HD are all vinyl and we did have a bad experience with them. My husband accidently hit his boot against the frame of the patio door and a 7" long piece of vinyl broke off. To our amazement the piece was hollow, not solid vinyl. Jeldwen would not replace the door even though it was brand new. Said it was our fault. We think its a design flaw. If you're going to stay in your home and can afford it, I would recommend Andersen. They make a retro window but we've only had experience with their new construction 400 series casement windows and their 12' Frenchwood Glider (it looks like a French door and not a patio door, i.e., 9" bottom wood rail). I also love their screens. They have strong mesh (not the normal fiberglass cloth) and you'll love the way they easily clip in and out for cleaning (we have casement windows). If you want to learn more about dual pane windows, check out my post "Andersen glass failure due to negative pressure". I still think they make a great product! Good luck with your project.

French doors usually come "pre-hung" which means they are installed in the door frame already. To do it right you need to buy doors that match the existing opening or alter the opening, including installing a new header and framing for the pre-hung doors, to fit. Altering the doors to fit the existing opening is not a good plan.

I was never a fan of slider doors and agree that going back with a swinging door is best. Two 36" doors would be my first choice. If swinging at either end won't work,one can be swung from center. Alternatly,you can install two fixed plus one swinging door. You can choose single pane glass with or without deviders in 1-2 or all three positions. Two 24" slabs fixed at each end plus one pre-hung 32" hinged at center position equal's 80" of door,leaving 16" for jambs and trim. The configurations are endless if you hire a good carpenter.