I see you have been around.
The metal cans will be better. The plastic ones can cause static, with the resulting sparks being no good. The metal cans might only be available in 25L though. a bit big if you do not counter balance it with something on the other side. As you use the fuel, the fuel side will become lighter than the balanced side again. If you can get 2 smaller metal ones, one for water and one for fuel, might work out better. maybe 15L each. Or maybe 2x 10L for fuel either side. It all depends how you are going to pack. a few smaller ones will be better than one big one, as the fuel will start sloshing around as you start emptying the contents. It is like having a passenger that keeps shifting around. No fun.
I have travelled through Tanzania, Uganda, Namibia, Zimbabwe and Botswana (4x4) and a 200 to 300 km range there would be enough. In Zimbabwe you would need enough fuel to ride right through the country, might be similar in other countries. You would need to carry extra fuel, and in depending where you go, even 40L might not be enough. If you stick to civilization though, 30L will be OK.
If its stupid, but it works, it aint stupid.