I see so many french door installations where they went inexpensive and bought a wood door, instead of a metal clad door. The wood doors always seem to deteriorate because they are never maintained, or they are installed with a Southern Exposure where they get full sun and rain.
Consider an aluminum clad door, you can get them where they are wood on the inside if that's what you want. Pozzi makes a good one, as does Anderson.
As far as the installation is concerned. You won't really know what you have there until you pull the drywall. Be careful to just pull the drywall, as it is common to have electrical wires running in the wall below the windows. You may also discover plumbing drain pipes or supply lines in your project area. These can almost always be relocated, but you have to remove the drywall.
At any rate, chances are that your window rough openings are not as high as the rough opening for your doors, so you will have to reframe the header above the door opening anyway. I also suspect that you will find two separate headers above your windows, with a full size stud or two separating the headers.
Plan on having all of the drywall out between the floor and ceiling in your project area. Depending on how tall your ceiling is, you can also explore installing a transom window (a narrow window above the door) and then the header above that, if you have room. This can be a very exciting touch.
Be careful on the outside to not cut away too much siding. If you do this right, you should be able to install the door, nail the exterior weather flange to the home's sheathing, then cover the flange with brickmoulding and paint and caulk it. You want the cut in the siding to be just wide enough to fit the brickmoulding.
I hope this helps, I am kinda rambling this morning. Time for a cup of coffee.