Mind if I give it a try?
Try to find some motors with a lot of torque, and a motor controller with PWM that can handle a good amount of current,2-3 Amps should be plenty.
As for the pressure sensor, try and get a digital body scale, (cheap one) and take out the sensor inside. You will need to calibrate the values from the sensor, 0 Lbs - 20Lbs (something like that). Those values will determine how much voltage in "PWM terms" to supply to the motors. You also may want a contact button or switch for when the person is trying to pull the suitcase. This switch will enable the motors to move when needed.
The rest is up to you.
Code wise, there are MANY samples for motors, sensors, LEDs, switches,etc...
The arduino comes with those types of examples, you just have to combine the ones you need into one main program.