Carol,

On safari if you are shooting into the bush you will probably use manual focus as auto focus may focus on the wrong aspect of what you are shooting AKA the trees in the foreground.

You will also be shooting in either morning or evening light so make sure the lens is fast enough. However bumping the ISO on the camera will help a lot in low light.

Also get a large enough card and someway to download the card during your trip.

I would go with the doubler rather than the 500mm. Less weight to carry around.