A123 technology for a cheep HW robot is A bit unusual, I use them on Cherub and Behemoth team use them too but we have an 8s4p configuration keeping the volts around the 24v mark. you could use SLA for this as for low cost HW these are cheep and I have used them for years in my heavies. If you want to get a heavy in the arena I would advise something like this.
2* 12v 16amp/hr SLA batteries charged with a car charger!
2* Botsbitz 85A speed controllers (I believe they will just about go to 24v) http://www.botbitz.com/index.php?rou...&product_id=59
2 24v wheelchair motors. Try to keep the gearing low so the thing has traction 4-5mph doesn't sound fast but it means you might be able to push the other robots about.
Make the robot small so the mild steel armour can be thick at least 4-5mm. and that will get bent! try to make the robot look different and fun. people have spent their money on tickets to see it! make it so the robot can be repaired in the pits easily. you will find a lot of teams make it so the whole body lifts off on a few bolts so access to the parts to be fixed is easy, shock mount everything you can.
Other things to look out for make it so it doesn't get stuck on the arena floor it is not flat or on the side. if it can't self right it must be invertible. Don't get hung up on weapons to start with, in this game there is a steep learning curve if you can make a robot that can get into the arena and drive about with out getting stuck or breaking and then driving back out of the arena, that's about the best start you can hope for.

I would encourage anyone to have a go, remember a lot of money thrown at a robot doesn't automatically make a good machine, design it well build it well and you with a bit of luck might win.

Don't give up

Craig