Off road navigation has nothing to do with road navigation. A road GPS navigator does not allow you to enter coordinates, tracks, or waypoints, so you will not be able to use the standard GPS utility vehicles incorporate to participate in the Tunisie Challenge.
If you do not have a navigator, you must buy a traditional GPS (GARMIN). Click here to see models.
You can also use your SmartPhone (preferably large screen) or Tablet (preferably 7 inches) as long as they have built-in GPS. For these systems you can use two types of navigation software, TwoNav or OruxMaps.