Hey Andrew, why not use standard drill motors for the moment? they are very cheap and easy to use and you get batteries that you can test it on with the drill. Also for the price you would save on buying gold motors and seperate batteries, you could afford four motors!
If you need ANY help with building or designing then Rc Wars/Robo-Challenge will be able to give you help and advise. We build many robots (6kg to 12kg) and have a fleet of just over 15 at the moment and so we would be able to build any components for you if you do not have the facilities. Where abouts do you live? we have facilities for testing your robot aswell if needs be, and supply electronics if you are uncertain of where to get things.

Grant