I looked at those motors but I'm not sure if they're a little underpowered or undersized for a kilobot. So any experiences/ideas of how they might perform would be appreciated
The only experience I have at that weight range is super-antweights, just using servos and receiver battery packs. So I'm trying to imagine something four times the weight of that. Servos might cope but would be slow to move unless speed-hacked.

I was also looking at the Technobots range of MFA motors, like the ones used in Zeobots/Battle Ratz but some of the smaller versions. I remember seeing a few machines using them at the UK Robotic Games back in 2006 for the 3kg sumo class and they were quite a good speed with what seemed like decent torque. Haven't looked properly into the weight of them yet, but compared to those 6V ones Craig linked to they'll obviously be heavier. Also means the possibility of going overweight more easily, as the bigger motors will need bigger batteries.

But if those small motors can cope with the weight, could mean plenty of weight for a decent chassis and weapon. Several things to consider *ponders*

EDIT: Just seen Calum's post, so if they are fine then that could be a six-wheel-drive lightweight chassis sorted :-)