It really depends on a couple of things, the bad things you know about him are they really his personality or were they things brought on because it was 2 people who really didn't belong together. There are some people who really shouldnt be together and they bring bad things out in each other.
How close were you as a girlfriend, were you best friends who drifted apart because she was overwhelmed with her life and if you called her tomorrow would you pick up where you left off in the friendship or was it a more casual friendship. If you could pick up as best friends where you left off if you called her today I would talk to her about it. If not I would do what your heart and mind tell you and try dating him.
I have a girlfriend who was dating a guy and his best friend lived with them. Well one day the best friend moved out and asked her if she was going with him because getting to know her he had fallen in love with her. They are still happily married and have grown kids.
If you decide to date him just make sure you always communicate how you feel about things. Also you may have to look at it like you just met him off the internet so you don't let other things interfere.
I hope some of this helped.
Have a great day!