Gardyloo is correct. If you have luggage carts, you can use these on the escalators in the garage, too.

Can't speak to the GPS units being up to date, but if you don't know your way around, you should definitely get the GPS, especially if you're sticking to the Joburg area. Are you driving around the country, or staying in the Joburg area? You wouldn't need a GPS for some of the longer drives, however in the larger cities you might find that they're lifesavers (sometimes literally - not being hysterical, but there are a couple of neighborhoods in Joburg that you'll really want to avoid).