Yes, Sal makes a really good point, however as you will be there in July, viewing will be fine, as the grass is generally lower. The thing I liked about driving a lower car, was that I was at eye level with a lot of the animals, which was great for pics - especially the lions and hyenas. The first photo below of the lion, (in rubbish low light conditions at 0500) and you can see how it was at eye level with me as it approached my car.
Regardless of which transport you choose I am sure you will really enjoy it. As it's the holidays there, perhaps you may want to consider taking more of the dirt roads, which generally are not as busy as the tar roads.
Ref the elephants, yes give them plenty of space, especially when there is a young one around. That said, they usually give good signals, and you will soon know if it's not too happy. Just make sure you have room to reverse. Another tip, the hyenas have a liking for chewing car bumpers etc! So watch out if they come too close, and suddenly they are out of view, they are more than likely chewing your car bumper!
Regarding closing the windows, I was with a pack of hyenas for a good while last month, on a small dirt road, they were running around and generally being, well, youngsters. They were a respectful distance from my car and I was merrily taking pics, when suddenly, I looked down, and there was this young hyena sat right next to my car, looking at me, within inches of my window. (which relates to what I said above about the lower aspect of my car). That window came up pretty damn fast!
Another good road to travel is the southern dirt road, between Croc Bridge and Malelane (the S25), good for predators, and you should see raptors too, as it runs along the Croc River.
One last suggestion (!). Spend some time at Biyamiti Weir, it's a great place for photography, as you are eye level with the water too. (I understand a leopard is a frequent visitor, but not when I was there).
As I mentioned above, if you change the gates you enter/exit, this will give you a great chance of covering more of the southern park. Marloth Park is really well located, especially for Croc Bridge, as well as Malelane, and it's a really easy drive to get to both.