Searching for Free or Cheap School Library Cataloguing System

I recently was hired by a small local private school to completely reorganize their library. They have no existing catalogue or inventory list. I am in my first semester of SLIS and haven't taken a single course that would help me know how to do this (I told the school this, but they weren't bothered). I'm getting help from my professors but right now I'm trying to find a good cataloguing system that isn't too terribly expensive or difficult to implement. Preferably something that can scan ISBN's so that I don't have to create new barcodes for the entire collection they already have. I'm writing up a project proposal of expected time, budget, etc, and would like a few different options to give to administration. I'm still counting the collection but I'd estimate that they have around 6000 books. I'd be grateful for any suggestions!