Did I make a mistake by refusing a suitor who does not pray in congregation most of the time and listens to music? Is Istikhara permissible in this clear-cut matter? And how many times should Istikhara be repeated?

The sister is advised to marry the man if she can influence him to adhere to his religion, abandon music, and commit to congregational prayers. Otherwise, it is better to leave him if she finds an alternative. Her refusal to marry him due to his abandonment of congregational prayers (if he has no valid excuse) and his listening to music is not sinful.