I bought an 8 lb medicine ball, and it's nice, but honestly, you can just easily substitute a dumbell. See how it goes, you may not need a medicine ball. I don't really need it, but since I have it, I use it. Now the stability ball is another story. That really is a useful tool and rarely can be substituted with a dumbell or a bench.
The medicine ball comes in 10 lbs also, for what it's worth.