In Cape Town June is winter and the rainy season. September is spring, therefore much warmer, but still the chance of a days rain exists. But not enough to ruin your 3 week trip.
com/ is a site where you may see what the weather holds for the week across the whole country.
There are many speed camera's, on national roads the limit is 120kmph, but when passing through towns or roadworks drops to 60kmph. They are not shy to trap in these area's - as previously stated in thread, is a source of income.
Most area's of South Africa are very safe, just use a little street sense.
To those who say it can't be done - stand aside for those who are already doing it.