We had a rec on Thrifty from JP of Blue-Africa Safaris, the great guy who gave us a ride to Ngepi.
We used Expedia to check their rates, and according to Rob of Shoestrings Airport Lodge backpackers, the rate was great.
The car we got was a four door VW Polo Classic with a "nice boot," a trunk, not a hatchback, which is what we wanted. It also had a standard trans, which we prefer, and air conditioning.
The biggest problem with a passenger car for a self drive is its low clearance, and as we later saw, there were a few places in Kruger we could not go as a consequence.
However, there are miles and miles (kilomoters and kilometers ) of Kruger National Parke where we could, and did, go.