sounds like a plan you have there.
I wouldn't worry to much yet. Just go to Ghana, get a feel for the continent, and decide what you want to do. When you live there you are going to make contacts that will help you a lot, i.e. mechanics, expats etc.
Big bikes are not easy to come by and perversely expensive for what you get. I think you will be happier with a small Japanese bike (Yamaha DT 175?).
The carnet (or rather lack thereof) will pose a problem once you reach Namibia, but people have gotten through without it. Remember, in Africa there is always a way. Friends of mine without a carnet camped in front of a Nigerian customs post for 2 nights, had to draft a paper describing their "mission", get ten thousand stamps on it, and even talked their way out of having to pay.
The time is OK.
Just browse the HUBB, read some blogs (http://www.africa-overland.net
), look into visas (and where best to get them).
Don't worry about anything right now. For now, the best preparation would be learning some French. Trust me, when you're there, you'll be glad for every word you know.