Yep, a perfectly reasonable alternative to focus all your attention on a small area. Horses for courses! Can't say I much cared for St Lucia, though. My favourite part of SA is Bushmanland (Pofadder, Brandvlei, Aggeneys etc).
Some very good advice already. Dealing with a few other points:
1. Best time rather depends on any specific interests, but generally October - November is probably the best period. Kruger can get very busy, especially during school holidays; try to avoid these.
2. It's definitely best to book...