I have spent alot of time in Africa and have gone on alot of photo safaris. Are you going it alone or with an organized group? It makes alot of difference.
For your budget, I would get a 400 mm lens. A couple of things to consider: take a good monopod for using in the minivans. I think that would be preferable to a bean bag. Also, be prepared for alot of dust. You want to be able to cover up your camera and lens between stops and be able to pull it out quickly. Likewise, film changes can be dusty as well.. Clean your camera every day. Also, be prepared to be annoyed by the others in your group who constantly jostle around and vibrate the van and chatter with endless wonderment at the thousandth lilac-breasted roller. If you have the time and flexibility:
Show up in Nairobi, walk downtown, and arrange your own tour. My wife and I did that several times with no problem. We hired a minivan with a driver/guide and a cook/camp guard for a lower price than an organized group. We would camp some of the time and then spend some days at the expensive tourist lodges. Camping is really nice with hyenas prowling around and lions grumbling at night as you wonder why you drank so much water after dinner. Also, you have the vehicle to yourself and can spend as long as you want wherever you want. Not the ADHD pace of the tours. Whenever they spot a leopard all of the minivans take off in a cloud of dust and then create a parking lot around the animal.
The 400 mm lens will let you get some bird photos of larger birds as well.