Hi Arun
I'm a born and bred South African and Johannesburg (Jo'burg, to the locals) resident. I'm not sure how your currency compares to ours, but the cost of living isn't that high. If your cousin's job includes all those perks, he should earn more than enough to live on.
Most of the schools in the cities are quite decent, although some of the schools in rural areas have a poor reputation. Almost everyone that your cousin will deal with can speak English.
We have a beautiful country and wonderful weather, but the crime-rate is worrying; though
you should be okay as long as you stay out of the dodgy areas. As muse said, a car is a must. Our public transport network has improved with the introduction of the Gautrain (a train which runs around the Johannesburg/Pretoria areas). but it can't be compared to the US or the UK.
Lastly, yes, we have Indian food.