The Arduino can not directly drive a motor. For that, you need some kind of transistor to switch some external power supply that powers the motor. If you want speed control, and to be able to run the motors in both directions (helpful when the line backs up), then you need an H-bridge. Buying one pre-made is generally better than trying to build your own.
The pressure sensor is, I presume, to determine how heavy the suitcase is, so you know how much help to supply. I'd think that something in the handle to sense when the suitcase is being pulled or pushed would be useful, too.