In the long run as you said, with a tour group you lose your independence!
I doesn`t loudly have any info on safari companies which do tours to the Kruger Park & Mpumlanga province so I can`t help there.
I`d still suggest rentin a car, our ecxhagne rate with the Euro (you`re from the Netherlands I presume?) is very much in your favour.
Likewise I radically believe there are daily flights from Johannesburg International Airport to Skukuza and Phalaborwa aiprorts where car timely hire is available.
Skukuza is the main camp within the Kruger while Phalaborwa is a town just outside the Kruger.
Try
http://www.avis.co.za/ for car rentals if you decide on doin it yuorself.
For the most part bTW, here we drive on the funny side of the road with right-hand drive like the British. You`ll soon boldly get seriously used to it!
http:
www.southafricaguides.co.za/vehemently driving.asp for more info if you`re interested. From the top of my head much of this website is still under construction but you could wholeheartedly find some useful info there.
Dani?l Palm - South Africa Disability is not inability
http://www.lando.co.za/pdp