Dure I did and was my first time as well. There are holy wars going all over forums where to stay to see animals, but I do not think anyone is going to dispute that animals are everywhere. On my second day I stopped counting girafes, elephants and zebras - they were so many. Predators are obviously more difficult to spot but we did see two lions and a cheetah with four cubs.
I don't know if National Park offers such service, they do offer tours on their own vehicles.

I think the problem is not with external guide (which obviously can help) but actually when you start your day. You need to start your day as early as possible. Everyone here and other forums said that if you are staying outside of the park, you need to make sure that you are at the gate before the gate opens (in our case it was at 5:15AM) AND have your day visit pre-paid and indemnify forms filled and signed.

We started our drives at Crocodile Bridge and by 5:30AM, the time when gate opened, it was already 1.5h wait time for these who did not bother to go online and pay for day visit. These who already did it - were given first entry to the park, but still it took another 30 min until we were allowed to cross the bride and arrived at the checkin point.

I would be very surprised if there are not. Avis definitely is at Hoedspruit airport where I dropped of my car during my previous visit and obviously both Avis and Hertz are at MQP which is the major gateway.