For Kruger I rent a mid sized SUV-more for clearance than anything else as its hard to see over the bush in a smaller car
Always rent manual transmission as it is cheaper
I am now a semi pro at drivimg on the left side of the road using my left hand to shift!
Always get zero excess as US credit card companies will not cover you if you take out any additional insurance
For Kgalagadi and Namibia I used local companies and rented a real 4X4-Toyota Hilux or similar
Driving in sand can be challenging and having a fridge/freezer in the truck meant my beer would never get warm

However that level of car is not cheap so you have to factor that in