If its your first robot why not just use the nice simple drill motors and gearboxes? 6 of them will give you a fair bit of power with less hastle and also will be a lot cheaper than buying speed 900's. 4 speed 900's would cost something like £120-160 (not having looked at prices recently) whereas 6 Argos drills will cost you £60 and you wouldn't need to worry about adding in extra gears or anything like that, just whack them on to a set of Robochallenge wheels and you're away.