Small network for a small business

Gentlemen and Ladies!

There is a task: Set up a local network for a small enterprise (10-15 users).

Available: 3 HP-Compaq servers (rack-mount), dedicated line and zoo PC and peripherals.
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Requirements: Firewall, File server (shared folders), Mail server.

We share our experience: What software can be used to do this, while respecting the parity “price / speed of setting / complexity of setting / cost of follow-up support”?



UPD by comments:
Option 1 - Windows
Option 2 - Windows / * nix
3 Option - * nix

1B: What application software can I use?

2B:
- Mail Server: * nix
- Firewall: * nix
- File System MS Windows 200 * / XP Professional

3B:
* nix - platform and application software. What are the options?