I have no experience quilting with sweaters, but I think your idea is wonderful. Everyones advice has been good especially the ones who advised you to get a couple of quilts under your belt first. The experience would be very helpful. The only thing I wanted to add is that knitted sweaters can't be cut since they will unravel. Are you planning to use the entire sweater in one piece? You can unsew the parts of the sweater, but cutting the threads would not be a good idea. I am anxious to see what you make, but take your time, and enjoy the process.
Sue