Depends upon what type of wildlife you're shooting. But especially if it's birds or wildlife that is a bit skittish around humans (like wolves), I think the 300mm isn't far enough. I'm going to Botswana in July and I've been told to think of 400mm-500mm range. Also, you're likely to be in lowlight in many cases (wildlife comes out at dusk and dawn to watering holes) so a faster lens is a good thing. This is especially true if you're shooting birds.
Also, a general rule of thumb for focal length-shutter speed that I've found to be true with long lens is the shutter speed should be double the focal length. So if you're shooting a 300mm lens you want to be exposing at 1/600th or faster. That's especially true for shooting birds that are moving at a distance (like a bird of prey). And that means you'll probably find a monopod or good hiking tripod to be invaluable.
B/c you'll often be shooting around water, that means a circular polarizing filter would be a good thing. B/c you'll be in wet weather waiting for the shot (or it rains going to or coming back from the shooting location then something to cover the camera (and no, a cover for your backpack isn't good enough). Think of a rain sleeve (or a child's raincoat/waterproof parka and you snake the zoom lens through a sleeve and the body of the jacket covers your head and the camera body).