Go to millets or somewhere and get a proper one. They can give you good advice too. And student discount.
You need it to be as light weight as possible, otherwise it will take up your baggage allowance. I'd probably say a 2-3 season bag would be fine. Make sure you check out the comfort zone temperatures on it, as they can differ between manufactures a little bit. Basically, go for the lightest one which is also the warmest one which fits into your budget. You don't need a 4 season one however. A 2-3 season should be fine and is what I've got and I've been all over the place with mine (initially got it for travelling in Central America) and never been too cold or too hot with it. Mine cost around £60 I think but this was 3 years ago so I think they have come down in price a lot now. If you have a Decathlon store near you, I'd highly reccomend their Quetcha products. Make sure you also take a liner (a single quilt cover is a good low cost option) as sometimes it might be too sticky at night for you to use the sleeping bag, so you can sleep in your liner and sleep on top of the sleeping bag. Size wise, I'd get a big one designed for men as then you can really curl up in it and they don't cost anymore. You normally get a selection of sizes, they tend to be 170/175cm or smaller, and another one for people over 175cm but get the bigger one as then you can go right down into it on cold nights and stop your head from getting cold.
if its going to get cold (doubt it since south africa but u never kno) then definately get a mummy sleeping bag. more tailored to body so conserve body heat.
If you're talking malaria tablets you need a mosquito net. Even if the mosquitos there aren't malaria carrying, a net is still a good idea because you will be bitten by bugs anyway. Although I think you will be taking malaria tablets in South Africa. What kind of places will you be staying in? If you're going to be sleeping on the floor in huts or something you want a box style one which can either be affixed to string you've strung between two posts, or fastened to the wall. Or you can get self supporting ones which stay up on their own. Don't get one which are designed for having a bed and are canopy style with as they tend to have a different style of fastening (from my experience). You need to have a mosquito net, if you're staying in a tent the tent will probably have some sort of mosqutio shaft on it which as long as you keep it closed it is fine. But you have to keep it closed and try and keep out the mosquitos otherwise they'll just stay in the tent and bite you.