You don't say what size garage door you are attempting to replace, but typically I don't see french doors available in an 8' width. To attaint the width of the original opening, you need to fill in with either sidelights (windows) or wood and drywall and siding, or "". Depending upon what kind of spring your garage door uses, your husband may be able to demo the door himself. If the spring system is the simple spring-cable-pulley system, no problem. I won't do torsion spring doors, period. This is something you should leave to a professional door installer.
As far as installing exterior french doors, I recommend you go with a metal clad door (at least on the outside) because they are better built and stand up to the elements much better over time. French doors are especially difficult to install because the larger frame is less forgiving for being out of plumb and out of square. In short, if you have done doors before and you are comfortable with installing them, go for it. On the other hand, if you know nothing about it and you have to ask a ton of questions about it, experimenting on a $1,000 door is probably not the best way to learn.