>Do I set up an external ip address for my airport extreme or is there one for it already?
Depending on the service type (consumer or business grade), your ISP provides your 802.11n AirPort Extreme Base Station (AEBSn) with a Dynamic or a Static Public IP address. Most consumer-grade service get Dynamic addresses; with business-grade getting either.
>Do I need to setup a dns, domain name or somehow register this ip address via dyndns.org?
To access the AEBSn and their attached drives, you would need to know the Public IP address. Since a Dynamic address will change periodically, you would need to use a service, like DynDNS, as you suggested.
The other thing that would be required is to configure the router for port mapping/forwarding to send AFP (for Macs) or SMB (for Windows/Linux) requests from the remote clients in order to access the drives.
>Do I need to setup an appletalk protocol?
No, this protocol should not be necessary.
>Do I need to contact my ISP and make arrangements through them to have this work?
You could ask, but most likely they will tell you that configuring personal routers for remote access is outside of their service agreement for basic Internet service. If; however, you feel more comfortable to have assistance, you may want to consider a service like Geek Squad.
>Do I need to set up a Leopard server on a Mac mini for this to work...
No, neither would be required. This can be done with just the AEBSn.