Thanks for the tips Trev, will surely come useful.
For the idle shaft will use silver steel as that comes ground to size (6mm), I just cut to length with no need for machining. The output cant be made from silver steel though, for ease of machining I'm using a 1/4" drive extension turned down from the original 8.2mm to 8mm. That is because I need a square drive to couple the gearbox the the wheels.

One question regarding reaming before I purchase any drills, how much smaller should I get the the hole drilled? The final sizes of reamed holes shall be 14mm, 12mm, 8mm, 6mm, and 5mm. Would say 0.2mm in dia less be ok?