Most stadiums have power now, however I would recommend a generator. You'll wish you had one if you didn't do that first. There will be times where you won't have power, maybe a the practice field/lot or even a stadium that might not have the option. If you're able to swing it, I would definitely find a generator that is low in decibels.
I've done batteries before, I wouldn't do it. I would spring for the generator. Oh yes, also get a cart for the generator to be hauled in with a quality extension cord.
Mics, speakers, sound board -- all that can be ran through Romeo Music. At the bare minimum I would get a 16 channel. That will ensure a few wireless mics if you want, or handhelds, plus room for mics for front ensemble instruments.
I hope this helps you some.