giova,
You will require a 4x4 to visit Richtersveld. Sedan vehicles are not permitted. Some sections are very rocky and other areas sandy where one can easily get stuck without a 4x4 and even with one.
August will be a wonderful time to visit the park due to the flower season in Namakwaland from August to September.
Just some advice on the time. It will take you nearly a full day from Augrabies to one of the campsite, which means if you book at De Hoop you might reached it nearly at dusk. One can easily get lost at that time of the day and I recommend you to book at Sendelingsdrift or Pootjespram for the first night. Augrabies to Sendelingsdrift is 632 km, but when driving in the park you will only be able to cover 25 - 40 km/h and its not due to speed restrictions

Total distance from Twee Rivieren, Kgalagadi to Sendelingsdrift is 1020 km and quite a bit to bite off for one day.
I am not sure if you are going to camp but I would recommend De Hoop for camping. The first time I visited Richtersveld was in 2001 when there were not even ablution facilities. Currently there are no power source at the camping areas, except solar lights at the Wilderness camp.
The reason visiting the park is mainly for the landscape scenery. Two days are a bit short to visit most of the areas and I would recommend staying at least 3 days besides the day you travel to the park.
Good luck with your planning.