If you want to go with a TI-89,
here is a little serial port driver for it. The diagram and program should be everything, external of the control system, that you need.
To me, it seems that just using a serial character display, with an rs232 level shifter, if necessary, would be the easiest bet for displaying information when debugging the robot. It would be simple to connect it, and simple enough to program it.
Anyway, it sounds like a great project! Good luck!