Like a 600 and 400mm f2.8. they will cost over $20k, but you are only doing this safari once, right?
You can sell the lenses when you get back. I haven't priced renting them.
From what I've read.most if the wildlife are too far away and the lighting too harsh for a consumer zoom that tops out at 300mm and f5.6.
If you aren't photographing, I'd invest in a quality pair of binoculars.
Have fun, and share pics when you get back.