There are two ways you might want to use a GPS for off-roading:
1. To have a base map of existing roads etc for visual navigation purposes, which you would be able to rent from a car hire company. These systems are designed for in-car use, and assume a level of stupidity which means that you cant normally get useful things like coordinates out of them, and are only really a visual display.
- To use as a proper navigational tool where there are no roads. If you plan to use an off-road guide like the Explorer books, then this works on coordinated way-points. An in-car GPS cant do this, because you cant input coordinates, only named locations, roads etc. If you want this, then you need a navigation hand-held GPS. You cant generally rent these and will need to buy your own. Garmin is the best brand (IMHO).