The Namibian dollar is linked to the South African rand and international exchange rates are the same for both. Rands are accepted for cash transactions across Namibia, but Namibian dollars are not recognised in South Africa. You can pay by Visa and MasterCard in urban areas, but outside the cities it’s best to carry cash.
Expect to have to pay cash for local campsites, fuel in rural areas (and occasionally in cities too), and for roadside snacks and souvenirs. Tipping for meals and other services is not mandatory, but is typically around 10%.
Namibia is a very safe country, but petty theft can be a problem in the cities. Split up your cash and store it in separate places, and don’t keep large amounts on your person at any time. There are cash withdrawal points in all the major towns.