Better stick to AF lens, since it's gonna make the best out of D100. And you can still use it manual. For budget issue: why don't you look for Nikon AF 80-200 /2.8 ED first generation, I think they're already cheap now. Double with Tamron 2x AF teleconverter or else, you'll get 160-400/5.6. Remember that 1.5 factor is not extend the focal length, just 1.5 crop factor of the image.
Pinky Mirror Nikonians@Jakarta,Indonesia-- it's near Bali, if you don't know where it is