With an 18mp sensor (right?) I would be less concerned about maximum focal length than I would with lens speed and focusing speed. Unless you plan to rent a billboard you can do a lot of "zooming" using a competent image processor. On my various safaris I've found that low-light performance, along with the ability to snap into focus and freeze the action, are the crucial elements. If you're doing early morning or evening game drives, lighting is all-important, as is the ability to use a shutter speed that can overcome low light and/or vibration. Managing ISO is also critical.