There is a list of Xfinity approved Modem/Routers. Most likely if you buy the equipment locally they can tell you if it is approved by Xfinity. I am thinking about doing the same thing. They will work with your boxes just as good as the Xfinity rental boxes.
I have current the X1 wifi router and 2 wireless tv boxes. If I purchase a recomended replacement wifi router from an approved xfinity soure, will the 2 wireless TV boxes still work?