My 1926 house has a mix that is orginial to the house (love wavy glass). The living room, breakfast nook and dining room all have casement. The kitchen, bathroom and bedrooms have double hung (and the other bathroom has frosted glass slats that open and close). So I don't see casements as modern nor do I see a mix as necessarily a problem. That said if the whole rest of your house has double hung, I would probably go with that. In my house there's a kind of logic to the differences--"public" spaces at the front of the house have casement, "private" spaces at the back have double hung. Just one different seems like it might stand out more in a not so great way.

7'3" is pretty big for a casement. When open, that size sash would put a lot of pressure on the hinges & operator. If you are leaning toward casements, and don't mind tearing the old frames back to the studs, then you might want to consider casements with transoms. Most manufacturers have the ability to provide detailed drawings of window combinations, so experiment with styles that might interest you.

The use of mullions in windows should be a carefully considered design issue. Aside from the need for them in the exterior composition of some kinds of houses they can help strike a balance between a sense of openness to the outside and a sense of enclosure and comfort especially in the storms that come with such open views. To me a balance is appropriate unless one condition is more important than the other which I have yet to encounter in single-family residential design. When there is a great view of the ocean or the mountains it is usually a horizontal view so horizontal elements in wide windows reinforce that idea and mullions in the upper sash are usually seen against the sky. If the double-hung windows cannot be tall, Cottage Style windows raise the horizontal rail as shown in the photo. Also, owners usually want to see the view from an exterior space like a deck and since casements interfere with safe use of a deck I don't find much opportunity to use them except for basement emergency escape windows and as double operating French casements but I stopped using those because of hardware problems. I usually use multiple awning windows above kitchen counters and in bathrooms because they look better than casements when open and can provide ventilation in the rain and they collect less detritus between the glass and the interior screen. Screens are another issue - they are less noticeable on the outside than on the inside and double/single-hung windows can have half screens that allow mullions in the upper sash to be visible from outside and be virtually invisible from inside.

Three of the vinyl windows are 26.75in wide and the other four windows are 21in wide. The reclaimed casement windows are 22in wide. So they're a bit narrow for some openings and a smidge too wide for the others. Height-wise, the reclaimed windows are 2 inches taller than the vinyls. There are other bungalows from the same year on my block, and those that have their original windows have casements in the front of the house. So it's possible that we're actually returning to a more original look. Tell me more about the flashing. I have other casements (different style, though) that we restored, and we weatherstripped them. There are storm windows on the outside. We haven't had any issues with water with those...Is there something else you'd recommend we do differently with our new window project?