Nelspruit has an international airport now but its at Whiteriver which is much closer to Kruger so thats cool.
Why not enter the park at a Southern Gate and drive through the animals rather than the pot holed and somewhat dangerous roads on the outside??
Kruger gate or even closer Numbi Gate are real close to the airport.
You could then stay the night at Skukuza or any of the other rest camps in Kruger.
The public self catering sites are great, clean cheap and friendly.
A bit further to the South is Berg-en-dal, the south is where you ar emost likely to see Rhino.
I'm waffling the question was how long to Orpen 2 1/2 hours max speed 120km watch out for the cows and potholes though.
If you want to be really senic travel up to Sabie, take in the Blyde River canyon and Gods window and then go via Klasserie to Orpen.
Enjoy your stay
G