Andrew,

Have you considered the Zeobot motors that they have at technobots. You can get these with gearboxes already fitted. They work well with the electronizes. I have just finished a 4 wheel drive bot that uses two electronizes (one each side powering two motors each)

Have a look at http://www.technobots.co.uk/en-gb/dept_528.html on the Technobots site or give Paul at Technobots a ring/email.

Ian