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Why do you, your companions, etc want to go to SA?
The answer to this question will help a lot in deciding on where specifically to stay.
Where you are flying from will also help in deciding if Cape Town or Johannesburg works best as your first destination.
I choose to use a travel agent to book my time on safari's, in country transport, activities and countries. I picked a couple and let them come up with a plan based on what we wanted to accomplish on our trip, and what broadly we could spend in dollars and time.
Have you done a search on "trip reports"?
My report explains how we decided and where we went.
Our African Safari (s)
Good luck