central linux server with single-authentication

Hi,

I'm searching for a solution for a centralized server-system with
single-sign-on with linux as operating system.
I just want the following services:

- fileserver with client-authentication
- printserver
- proxy-server (squid)
- VPN-System

It's important that there is only one one central user-basis to
administer that can be used for every service on the network. Beyond
that it's important to be able to extend the enviroment with additional
services which can access the user-basis as well even if they are on
another server.

Is there any cost-free (and easy to set-up) solution for this scenario?

Thanks in advance,
Junialter

.