low end/small business file server

I had a client, a small one, w/c had only about 15 computers. And they this accounting department w/ 6 computers and there runs most of the database programs like payroll, receivables and a lot more. they had an unstructured peer network.
the company is planning to have a file/print server, but they want only a low end one.
can anyone gave me some idea what kind of computer im going to setup and also what OS im going to used.
thanks a lot…..