Hi More Boost - Welcome to PCHF!
The internal nic's ip address should be in the same range as your internal network. Example: Configure NAT servers internal NIC to 192.168.1.250 - set DHCP scope to 192.168.1.10-240 Default gateway on the NAT internal nic is left blank.
You can set DNS to your isp's DNS, however this would not be my recommendation as it slows performance but it sounds like you don't have any other option. What version of windows server are you running? See the website below for more information. They have a lot of good information and tutorials which should help you decide which configuration options will work best for your particular requirements.
Networking Articles & Tutorials: Windows Vista / 2008 / 2003 / XP
Configuring Windows Server 2003 to act as a NAT router
If you have any other questions or would like detailed instructions on how to set this up properly post back with what version of Windows server you are running so I can assist you better.