If you use Encore, you can have the pre-made pieces of video as their own timelines, then insert just the new pieces, and have the end actions make the timelines play in the correct order, or if using Encore 2, use a playlist (I have not tried this yet in 2.0).
Making the DVD directly out of Premiere may save a little time, but with what for you is a hassle.
The hassles of using Encore is you will need to repeatedly clone your project, manually re-link the new video files, and then either burn out of Encore or make ISO files to burn later, or make the ISO, copy to a networked machine and burn one disc while starting the next project.
Mike