Awesome, man! You'll LOVE it, and I dare say, you'll be heading back. For my first hunt, I lucked into a series of very fortunate events, purchasing a hunt at a benefit auction. Included were several animals, and I too was torn on what to add.
Although I had some ideas (and a budget), I am glad I took the advice of the outfitter and PH. "Take what God and the bush give you".
If you've never been, you will simply be astounded by the varieties of game there. It's a good idea to have an idea of what you would like to pursue going into your hunt (and be sure to discuss these with your outfitter PRIOR to your arrival), but be sure to keep an open mind. You may see something that was never on your radar, and you may even be presented with opportunities never imagined. I've mentioned this on threads past, but I was fortunate enough to have the opportunity at a good, mature cape buffalo on my first trip...something that I didn't think I could afford, and certainly didn't plan on.
So again, keep an open mind.
However, if I were to choose a couple of animals, they would be those endemic to, and readily available, in the area you are going.
In addition to your impala, I would consider (in no particular order) warthog, zebra, springbok, blesbok, and a wildebeest. These are typically readily available, iconic, and relatively affordable. Also of note (in my mind, at least) is the fact that black wildebeest are only available in RSA, to my understanding.
Best of luck, and I look forward to hearing of your hunt and seeing pictures!
On a side note, the wonderful folks here are a wealth of information. I did much of my pre-trip armchair research here, and it really helped me prepare. Since then, I have had the good fortune of hunting over there on several occasions. In doing so, I have learned what works for me in terms of gear and such, and, just as importantly, things that are not needed. I thought this might be useful information, so I put together a document containing this stuff. If you are interested, PM me, and I would be happy to share it with you. It is by no means a comprehensive list, but again, just some things I have learned through combing over threads on here, stacks of books and magazines, and from a little experience.