Small office network

I need to establish a small office network. Up to 5 PC running word processing, spread sheet, accounting such as Quick books, project, access to the internet and other miscellaneous software. Other than the basic pc’s what should I add to my shopping list to get my network running. I am looking for ideas on expansion cards, do I need a router? Do I need a seperate firewall? Will peer to peer work? Your input is appreciated.