The laser cut parts have arrived and I've finally worked out how to build the steering mechanism properly - seems like the steering shaft itself needs to be beefed up on the lower side to sustain the leverage forces created by the offset centre of the wheel. The shaft starts of as being 25mm and then eventually steps up to 50mm at the point where it is welded to the plate on top of the transmission gearbox, at it's point of maximum weakness.

The downward forces on the bearings are not great so everyday ball roller type should be fine as they will sustain an axial force of about 20% of the radial forces. I'm not quite sure what will happen when they start to where down as currently there's no provision for adjustment.

The transmission shaft looks similar to the above but is a straight 25mm OD as it has support bearings right up close to the actual wheel and consequentially much less leverage force compared to the steering.