You can get free domain name here:
http://www.dyndns.org/services/statdns/ 
As for the NAT, I use typically a router or firewall for that (hardware based, not software). They are usually safer
As for the internal net, you can use an addressing scheme from any of the reserved ranges:
Class Networks
A 10.0.0.0 through 10.255.255.255
B 172.16.0.0 through 172.31.0.0
C 192.168.0.0 through 192.168.255.0
Since you only have a few hosts, I would just assign the ip's manually, i.e.-
Computer 1 : 192.168.1.1
Computer 2 : 192.168.1.2
Computer 3 : 192.168.1.3
etc.
Have them all point to your NAT device as the default gateway
And here is a good article on how to use win2k as a nat device with ICS:
http://www.webpronews.com/it/network...NATandIAS.html
Hope this helps some
/JD