Welcome to the forum, safari holidays are always a bit special and Kenya is right up there among the best, I'm so jealous .
Lens wise you need to take a long as possible and then a bit more, 300 mm is an absolute minimum, 600 mm + is better.
Also consider hiring this specialised kit you going to need for the duration rather than buying it. Also remember to make provision for some sort support system, it will be needed for these lenses they get kinda heavy in use after a while. It might just be a beanbag, monopod, tripod or something, the organised tours may offer loans of this type of stuff, you'll need to check it out with them.
Above all else enjoy you trip.