PMS and PlexConnect need to run on your laptop , I guess. But if you set up PMS correctly, it should be able to grab the data form the hard drive connected to the router.
Set up the hard drive as a mounted network drive, set as PMS's central media location and you should be golden. I would worry about the constant and frequent writes and read to an external drive like that, as well as the speed, but you should be fine for a couple of years.