With a single 2500 and fibers would not accomplish that. 2500 is a low-end router, which has gone end of sale. So I doubt if you will get enough cards and module for that.
You can do NAT on the router to convert private IPs to the public IP you have got. Also on the router you need to make a username-password database and also need to define logical interface where users will land when they dial in.
But again, 2500 is not meant for ISPs so think before whatever you do.
Best of luck.
This was first published in July 2003