You will only need to invest in a server if security is important to you, you have more than one user per PC or you need to run server-based applications (e.g. SMTP server, SQL database server).
The main issue is security. If you have many users per computer, you will need to set up profiles for each user on each PC. This can be time consuming. A server will give you a centralised user administration function.
Another advantage of a server program it is optimised for sharing files. It doesn't sound like your users will be transferring files at a rate where a server would help.
If you do decide to invest in a server, have a look at the Microsoft Small Business Server product. This is the standard Windows 2003 Server software, bundled with a good integrated firewall and Exchange email server software.