Awesome! Welcome to the forum!

You sound like you have quite a lot figured out. Do you have ESCs yet? If so, which ones?

I wouldn't be so scared by the LiPos. As long as you treat them with respect and follow the safety recommendations (which you should be doing with all parts of robot building), they are no more dangerous than a normal battery. Regardless, the NiMh will be a good start to test with. With regards to the weapon, an axe would probably be easiest for the design and setup you have.

Look forward to seeing it develop and eventually seeing you on the circuit!