It was the System I was planning on going with originally. One of their scourcebooks has a small section on using the rules for cops vs gangsters. The important bit from there was the rules for modern guns, although you might find better and free rules online. Low Level Super abilites could be inserted via their skill system. Mordheim had advances that gave a character +1 strength, which could take them over the human limit of 5. A vigilante warband could have one or more characters starting with these abilites, almost like the mutations from Mordheim. If you get a chance, check out both books and see what would be usefull to you.
For myself I'm using a variation of the Chaos & Supersystem rules set. I'm rather enamoured with the Henchmen and character rules.
Hmm yeah maybe
Might just go with the Masked Vigilantes Idea
Cheers
Steve