Is LEGO mindstorms good for a robot warrior or a small size soccer robot?
An option that comes to mind is building a Raspberry Pi-based robot. But it's not likely to be small nor easy to put together, code and all. 

I think going for a LEGO robot would be easiest; motors, electronics and power source have all been taken care of. You "only" need to connect them and program the EV3 brick. There are numerous YouTube videos of Mindstorms soccer robots that can inspire you.