I recently started a similar project. Perhaps I can help.
I am assuming that the computer running your server software is one computer on your Airport wireless network. If this is the case, my comments would apply (I think).
1) Before setting up the server, use the Airport configuration utility to reserve the Server an IP address. The server needs to have the same IP address on your network each and every time. I had intermittent problems using Server Preferences until I did this.
2) After you decide what IP address your server will have on your network (I use 10.0.1.50) then install the server software with this IP address, and make sure the machine settings reflect manual IP address. Since it has already been reserved in the Airport Router settings, your machine will always have this available.
3) Unless you pay for a static IP address, you will get a new address periodically from your Internet Provider. I use dyndns updater (
http://www.dyndns.com) to address this issue. You run an application on your server which talks to their servers, regularly updating your external IP address (the one your provider assigns you) on their server. Users type in an address you get on their site, and there requests are redirected to your current numeric IP address.
4) Without any additional settings, my Airport Extreme sends all the traffic that comes into via my external IP address to my server.
Hope this helps. Mine is a work in progress, so any comments are appreciated.