Help Please

Im takeing on the venture of Creating a network for a friend of mine for his buisness. He has a 23 – 26 Pc Setup, I plan on using Windows 2003 SBS With Exchange and ISA Server. I plan on having the users on NAT, I also plan on having 2 Server pc’s. I will have a website on the second box, and i will also want to have the internet send email to the exchange sevrer. Here is my concern. With all thease servers visible to the internet, how can i prevent hacker activity?