16 years ago, my friend and I were planning to ride Africa. He ended up riding from Dakar Senegal to Cape Town and I opted to ride across Canada. Canada was great but I regret not going against my better judgement and joining him. Every post card and audio tape he sent to me was like adding more salt to a wound. I haven't read all of the posts but what you are doing is sane. People who don't think this is a good idea probably haven't been to Afica and are making their recommendations based on the media. If adventurers listened to all of the naysayers, we would all be staying with civilized touring in North America, western Europe and other developed countries. My most memorable tour was in a third world country and was more rewarding than anything else that I have done. Having said that, of course what you are doing has greater risks but the rewards will be greater.
Anyway, my friend did Africa on a $360 mountain bike and the price included racks, however I wouldn't recommend this. This was also his first bike tour but....he is one determined individual, intelligent and able to endure hardship with a smile which more than compensates for his lack of touring experience.