I was going to do the figure and all the accents (just weights and a gym bag in my case) in modeling chocolate but I'm wondering if it will be strong enough for the weights. Would you do all the accents in modeling chocolate, or would you do some in fondant, and which ones? Thanks! My main worry is just that everything holds up and does not break.

I think if you keep it cool you can expect it to hold up. It does get soft when it's warm, though. I have to keep taking a break from it when I'm using MC, because I have hot hands.

That is a cute design! I would use modeling chocolate for all of those little elements because it allows more time for sculpting than fondant. Work patiently in a very cool, clean environment.
Good luck with that cake!