Stephan,
It doesn't matter if you use an IP address or an interface on the 'ip nat inside' or a port. On the routers where I have this set up I have the live IP address configured on the loopback 0 interface and so my NAT statements use that interface so that the translation uses the IP address of that interface. If your external IP address was on say interface atm0.1 then you could use 'atm0.1' in your NAT config. If you were using another IP address that wasn't configured on an interface then you would use an IP address. Generally, if the IP address that I want translating is on an interface then I'll use that interface in the NAT statement.
If I want to only allow certain IP's to connect to the external address then I'll put an access list in place to filter traffic to my router. I generally configure NAT mappings and then also have access lists to filter and monitor connections.
Chris.
**********************
Chris A.C, CCNA, CCSA
**********************