If you have spent any time on Mumsnet, you must have known what response you would get. There is no account given on here for the fact it is possible for some people to change, absolutely none.
All of these things were true of my dad when mom met him, apart from the drugs. (He did abuse alcohol). He spoke ill of his ex because she was a violent drunk herself - he has a scar on his abdomen from a knife wound she inflicted. He turned out an excellent father to me, and an excellent husband to her.
People CAN change. It very much depends on the circumstances that led to them making the choices they made in the first place.
OP, you haven't given us enough information to judge. The one that would concern me most here are the fact that his drug and alcohol issues are recent, so it's hard to know how secure he is in his sobriety.
What worries me (and I don't mean this unkindly) is that you don't feel able to make a decision on your dating life for yourself - you seen very susceptible to the opinions of strangers on the internet. I wonder if you are easily influenced by people in real life too?
So maybe take a break from dating, especially serious dating, and take more time to heal, so you become confident to trust your own judgement.
(Says a stranger on the internet. The irony of this isn't lost on me!).
Good luck.