A lawyer for the city estimated that Sept. 11-related lawsuits brought by firefighters could cost up to $12 billion. The city's Law Department has set up a special World Trade Center defense unit of 21 lawyers to handle 1,700 cases stemming from the attack on the World Trade Center, Michael A. Cardozo, the city's corporation counsel, said at a City Hall budget hearing Friday.The majority of the claims come from firefighters, who, along with some police officers and sanitation workers, say a lack of proper respiratory equipment during the recovery and cleanup of ground zero caused health problems.Cardozo said the cases would last for many years. He said the special defense unit had a budget of $3 million and was creating a database for five million to seven million documents it will use to fight the lawsuits.