Hi Juan
Gee, lots of questions! I'll try to answer a few.
As to buying a bike - you might have more success in Johannesburg than Cape Town because it's bigger, BUT Johannesburg is not considered to be very safe - carjackings, very bad crime situation. Also, we do have contact info. for Trefco, a BMW dealer in Cape Town that has been of assistance to a number of travellers including us, who you could contact in advance to let them know your plans. They may be willing to agree a buyback price when you finish your trip, or sell the bike on consignment for you.
As to route, we haven't done every country in Africa, so can't comment on some of the ones you mention. Our route was by air into Kenya, then through Tanzania, Malawi, Mozambique, Botswana, Zimbabwe, Namibia and South Africa. Highly recommend Namibia for the scenery and the animals, Malawi for the people and some animals, Botswana is more expensive, Zimbabwe was great for elephants in Hwange National Park, also Victoria Falls is worth seeing, but possibly avoid Harare and the bigger towns. Tanzania has the Serengeti and Ngorongoro Crater, as well as Kilimanjaro, but the most popular national parks are hard to get to and expensive.
You can't ride a motorcycle into national parks in most of Africa, only if the main road goes through the park will you be allowed to ride through. So, if you want to see animals, that means find a place to store the bike and go on safari, or rent a 4WD or camper. Best value for that is Namibia in Etosha National Park, our single favorite park in Africa, as you can rent a vehicle and spend as long as you like wandering around. But I am animal crazy, you may not be!
For info and opinions on the other countries in Africa which we haven't seen, you could try Peter and Kay Forwood's website, they're the Aussies who are going round the world on a single Harley, and they went through just about every country in Africa last year.
http://members.ozemail.com.au/~forwoodp/
And, sorry, we've never been in Vietnam so can't compare. There is less traffic in Africa generally, but road conditions in some areas are appalling. Also, they do have a rainy season which varies from area to area, so that may make the roads even worse.
I guess the important thing is just to not be rushed. In three months you can do a leisurely tour of southern Africa, a brisk tour of south and eastern Africa up through Tanzania, or a frantic race to get as far north as Ethiopia and Uganda and back to South Africa. Or, if you didn't want to make the round trip, buy bikes you want to keep and ship them home to Europe from wherever you end up.
Cheers
Susan
[This message has been edited by Susan (edited 02 November 2001).]