Are mobility scooters and wheelchair motors the same thing to you? All the wheelchair motors I have seen are around 200 rpm at their rated voltage, which seems slow to me. Unless you over-volt them and use large diameter wheels, they will probably be too slow. As Maddox already said, the other problem with eBay motors is finding replacements.

I looked at US heavyweights (which I am more familiar with) and they all seem to use at least 1HP motors. Using that as a baseline, you need at least a 750 watt motor and that G43-500 isn't up to it. When I started looking for larger scooter motors on eBay they were nearly all close to the price of the E30-400 Motor. As the scooter motors were all lower power and weaker construction than the E30-400, it seems quite a bargain to me.

What controllers are you planing to use for the drive motors? I seem to recall you saying that 24V was the highest you can go.