It's best to choose doors that match your home's period and style. But if your house doesn't have a discernable architectural heritage, you have more leeway to introduce a new look, as long as it's in keeping with the scale and feel of the space.
Be sure to take into account how much room you'll need to accommodate the door swing. If installed in a dining area, the doors will require enough clearance to avoid bumping into chairs placed in front of them, or into furniture, such as a built-in china cabinet or buffet placed along the wall where the doors will rest when left open.
The direction of the swing is also important, particularly is they lead to a patio or terrace. It's easier to install an exterior screen if the doors swing into the house. But to maximize interior floor space, you may want out-swinging doors instead. For the latter, just make sure your patio is covered, so wood-framed doors don't get wet if left open during a sudden rain shower. If it snows in your area, an overhang will also keep drifts from building up against your door, making them hard to push open.
If you plan to swap out your current contractor-grade French doors for higher-quality antiques, you'll likely have to patch the doors' old hinge mortises and cut new ones that line up with existing mortises in the jam. Adding sets of salvaged doors where only a single door hung before will mean resizing the opening. "You might have to put more time into the prep work with antizue doors," says Mike Thompson, sales manager at The Old House Parts Company. "But if you are looking for better craftsmanship than most new ones—they have true divided lights rather than muntins floating over a single large piece of glass—and something with more character than what you'll find at your home center, it's worth the effort.