KQ, I won't be able to answer your paperwork questions but I will try and answer some of the rest.
People leaving: People leave ATNS for reasons ranging from crime to lack of recognition. The biggest reason is for money which I will explain below.
Pay: Seeing that you will probably be employed on contract basis there will not be deductions for pension on your pay. How the tax system work for non-citizens/residents is also something I can not answer. I would venture a guess and say around R25 000 after tax.
Perks: Covered parking.
Overtime: Involved formula to calculate. Depends on basic salary. Time and a half for Sundays and 2X for public holidays.
Livingcost: In SA anybody can buy a house. If this is not your thing renting is a good "cheap" option. Interest rates are high and there is some cheap rentals out there to be had. For around R7k/m you can rent a very nice place. Cars are hideously expensive, so get something cheap but reliable. Depending on your taste, food is not to expensive. 2L of milk @ R15. Pack o' fags @ R15. Petrol @ R7/l. Beer @ who cares, its still cheap. Medical cover and insurance can put a bit of a dent in your pocket as you live in a high risk society. All in all you wont be able to live a champagne lifestyle and definately won't be able to send money abroad, but then your circumstances might be different.
Wife: No clue.
Foreigners: Loads of them. Serbs, Croats, Swiss, German, Danes, Canucks, even people from the East Rand
, the lot. So no problem there. ATNS is a para-statal so they must have some friends in Gov who help with these things.
People leave ATNS because of the money. The cost of living is not that high (my opinion) but the income in the end does not afford you the financial security that you want. If you have two incomes the picture will obviously be different. My advice would be to get some hard numbers from them first, post it here and then people can disect it for you. But whatever you get from ATNS, get it in writing!!!! They will renege on there word at the drop of hat.