Well, if you're looking for extra armourment, I would recommend using Perspex. Since it's fairly strong and less than half the weight of even aluminium, it's good if you don't have much weight left to work with. Also, I might recommend adding some extra weight to the end of the arm to give it a bit more momentum when you're spinning.

As for the heavyweight, how fast does that wheelchair go? I reckon if you want a competitive bot, it needs to be capable of at least 12mph. Do you plan on adding some extra battery power?