Hi Austin,
In terms of timing, anything is possible. I met a guy riding a KTM that left the UK and made it to Cape Town in 6 weeks. He didn't see much though. It's about what you want to get out of the trip. If 6 weeks is all you have, then you can make it work.
A few things to consider:-
1) flying a bike into Africa and then out again for 6 weeks is kind of expensive, not sure if you looked into this already. Chris at Jungle Junction in Nairobi can find a suitable bike to purchase and arrange all the paperwork for you.
2) another possibility would be fly to SA, buy a bike (exchange is all in your favour) and try and sell to another rider going in the opposite direction when you are done. Again Chris is fantastic at helping overlanders with these things as well.
3) renting a bike is another possibility - you just have to compare the costs though.
4) most NP's won't let you ride into the park so you would have to stay at camp sites or lodges outside and take a tour in.
And yes, everything can be done on paved roads with the exception of Namibia if you want to see the attractions. Planning is half the fun so don't stress too much about it. 
Daryll
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