Hi, I've also been looking into this.
From what I can find out, the Customs duty is 27 % and then VAT is charged at 12 %. This is based upon the declared value of the car (including, I think, transport charges from origin to Botswana).
If the car is being driven to Botswana then Customs will take 5% of the value for transport charges. There are no restrictions on age of the vehicle.
Hope this helps.