Hi Tony
I have not travelled the route from Jhb to Punda Maria (as I always come up from Durban), but the distance is around 550km, maybe somebody else can comment about the quality of the road?
Everybody will have their own "best times to see cats" opinion, I have always managed good sightings of cats in summer, including lion, leopard, cheetah, serval and caracal.
As far as camps go, if you stay in the main park (i.e. not the concessions - maybe somebody else can comment on these?), I think, as has been suggested, you should consider staying 3 days in the North (Punda Maria), 3 days in the central park (either Satara, Olifants or Letaba) and 3 days in the south (Lower Sabie or Crocodile Bridge). This will give you a good range of habitats, and a good overall view of the park. All of these camps mentioned are well worthwhile, and are nicely positioned for birds and mammals.
Enjoy the planning!