In my personal life I learned that sometimes when ppl complain about people to cover up something like to make you think that they hate the person and would never be with the person just to cover up.... from my personal experience
I'm with them ^^^ . there is no excuse to complaint to his ex. they obviously still have some kind of connection.
DONT MARRY HIM! I feel you and in the same boat if your gut is telling you NO! then go with it! You'll be thankful later on if things dont work out be strong girl your not alone :)
Would you want a son just like him? Do you want him to be the standard your daughters hold guys to when they're deciding who to date? If you died would you trust him completely to raise your child?
Don't marry him the only reason he would complain about you to her is to get her sympathy.... good luck... but I am sorry, I don't think he is worth the pain you would feel if he got enough sympathy from his girl friend and something happened.