I am going to assume that you are referring to the "Qatar Offroaders" event, and if so, you shouldn't worry. There will be 2 routes for the trip, one for experienced drivers, and an easier trail for new drivers like yourself.
As far as the cars are concerned, both will suffice and get you through the dunes/
Lowering the tire pressure is extremely important and you will most likely get stuck if you don't. The recommended tire pressure is between 15-20 psi. After you are done with the dunes, you can refill your tires immediately with a portable compressor, or at the petrol station right next to the dunes for 5qr.