As zoomkat stated, there are practical advantages of having physical cables on this type of robot. Another would be that wireless communication isn't going to be of much use inside of most metal pipes... Usually metal pipes either come into direct contact with the actual ground some where along their length, or are attached to enough metal and other material for it to act like a ground. A metal container electrically connected to a ground is one of the best ways to block electromagnetic waves, including radio frequencies like Xbee.
In regards to your ideas on the rest of the robot, your general approach seems to be reasonable. However, pipes often have bends or junctions, even the striaght pieces can be vertical or at an extreme angle. So you probably want a drive system that can traverse all these different configurations, as well as a horizontal length. So on a related note, I also agree with zoomkat that you should do search online. Not just for other DIY projects, but the professionally made pipe inspection robots, especially when it comes to overall structure and drive mechanisms.