Help with a redirect / splash page for sign up

Maybe not perfect for this list, but I gotta think somone on here has done this before. We just got handed over 500 DSL subscribers. Old ISP is dead, no records, no accounting data. Just the ATM PVC's are on our network. I'm trying to figure out how to do a one time redirect so that they sign up into our billing system and once thats done then they have internet access. I'm willing to pay reasonable $$ for somone that knows how to do this using open source tools and our cisco routers. Mucho thanks for the help.