Hey /asp/, can any of you guys give me some advice on medicine balls please

I'm an aspiring amateur boxer and I want to get a medicine ball so I can improve my training at home, only thing is, I've got no idea which weight to get. I'm going to be doing the punch drills, sit up twists, throws, all the stuff to train my power, so I think I don't want to go heavy, but I don't want to go too light that it will become less of a challenge quickly.

So which weight(s) medicine balls do my fellow fighters use? Any particular brand/style?

Money's tight at the moment so preferably not too expensive.

Bit of background, I'm British, 6'1, and 89kg.

Thanks guys