No bouncing required, so you don't have to worry about that. In one exercise (I think in Muscle Endurance), she has you squeeze the ball between your hands to work chest -- a child's play ball will work for that, or a volleyball or basketball, or an overripe melon, you just need something with a little give to it. But in most cases, she's using the ball to provide weight, mainly for core exercises but also for leg presses and side squats (in Boot Camp). Almost all of these exercises can be modified using a dumbbell, and one of the exercisers shows the modifications. I did get the ball and like it, but I first did the tapes using a dumbbell and my dog's Wiggly Giggly ball, and it worked pretty well. (Occasional weird giggly noises from the ball aside.)
If you're not sure about the ball, you can definitely do the videos first with a dumbbell and play ball, and then decide whether you want to get the medicine ball and what weight would be good for you.
Audrey