Qball,
Depends a bit what kind of snake you are looking for: anything so long as it has a rattle, or do you prefer smaller species, or larger ones?
Species to avoid: C. adamanteus, atrox, scutulatus, durissus, vegrandis, unicolor, simus, basiliscus, oreganus helleri. They are either very large (thus with a potentially huge venom yield - e.g., adamanteus, atrox, simus, basiliscus) and/or have particularly toxic venom that has a higher than average chance of landing you in a box (scutulatus, durissus, vegrandis, unicolor, oreganus helleri), and a track record of clinical cases to prove it. Also, some tend to be extremely cranky and potentially hard to control (e.g., some atrox, scutulatus and helleri).
If you like larger but manageable species, C. molossus is often very colourful, and most specimens are fairly calm and laid back. Of course, being fairly large, there is plenty of venom there if you were to get bitten. C. ruber is also generally rather placid, as is C. cerberus, but again, although they are usually calm, bites can happen, and are likely to result in a good dose of venom inside you.
Among the smaller species, there is a lot more choice - Sistrurus spp., C. cerastes, C. lepidus etc. are all smaller, nicely patterned species that are unlikely to put you in a box, as are many of the smaller Mexican highland species - I don't know what's usually available in S. Africa. Most important of all, remember we are talking about the statistical chance of ending up with a tag on your toe. Just about any front-fanged snake could, in extreme circumstances, cause a life-threatening bite, it is just a much less likely outcome for some species than others. However, if your number is up on the day, you COULD die or lose fingers or more to any rattlesnake. For these smaller species, be sure to get specimens that are feeding on small mammals, many of these species feed primarily on lizards in the wild, and can be difficult in captivity. Having to force- or assist-feed is one of the best ways of dramatically increasing your chances of finding yourself with a fang in your finger.
Good luck!