Possum Hopper - quickest options are Virgin, BA and SAA who fly direct to Jo'burg where you can get a connecting flight to Durban - so only one stop. Another option with only one stop is with Emirates via Dubai - but you end up with two long flights so it will take longer. Finally you have the two stop options (one in Europe and one at Jo'burg) with European carriers which take you to their flight hub e.g. Madrid with Iberia, Paris with Air France, Zurich with Swiss, Frankfurt with Lufthansa etc.
For prices, it's a matter of keeping an eye out for special offers which turn up fairly frequently and booking when you think you've found the best price for your option...
For Jo'burg stops I think it can be cheaper to buy the Jo'burg-Durban leg separately BUT that means that if your incoming flight gets delayed and you miss your flight you will have to pay for another flight. If it is booked as one itenerary, it will cost more but the airline will be obliged to book you onto another connecting flight for free...