The 'proper' way to do it is to design around the weapon and have the rest of the robot optimised as a weapon delivery system (unless your main weapon is ramming, obv). Depending what components you have access to when, your skills, and the exact design though, that can vary a bit (certainly for a first build on this scale I'd shift slightly more towards making sure it all moves!), but weapon first is the accepted design methodology.

I'd also leave a 240w anything in the garage, modern heavyweights will probably be needing double that *at least* in drive power!