mum.
Dueling Death Eaters is definitely very different than doing it for fun. That doesn't have to be the main thing you'd do for the Order, though the likelihood of it coming up increases about fifteen-fold upon joining. We could work on it, anyway. We regularly work on dueling skills for others already.
I'm definitely in no position to say it's a bad idea, and as risky as it is, it does give you access to a wider network and resources. Overall, I think I'd trust in the wisdom of mum's pro/con list here.