You're on the right line, although there is some disconnect in your post.
First off, if you've setup port forwarding on your router, and you've started the web server then you're pretty much good to do - other users will be able to get to your server by entering your public IP address in their browser address bar.
That's the trick, though - by and large your server doesn't know anything about public vs. private IP address, which is why the preferences give you your local address - this will only be valid for users on your LAN. Remote users will need to use your public IP address (check your router's configuration for this if you don't know it, or check http://whatismyip.com/ from the server).
That's the good news. Now for the not-so-good news.
The first is you say you're using 'Lion w/ Lion Server'. Which is it? If you're running Lion Server then the web server settings are in Server.app, not System Preferences. If you're enabling web sharing via System Preferences that implies you're using Lion (client) not Server.
That's not a huge deal, depending on how far you want to go with this.
The second problem is your desire for users to '.. be able to open the folder and download whatever they want'. Browsing file systems via a web browser is radically different than a normal Finder-type view (unless you enable WebDAV and give everyone accounts on the server, which is a whole different issue). While it is possible to configure Apache to server directory listings (so users can see a list of the files to download), this is off by default and cannot (I think) be changed via System Preferences, so that means getting under the hood and configuring Apache directly. There's also no file previews, icons, etc. and limited metadata - in other words, if you have a directory of images, people are likely going to need to download all the images in order to determine which ones they want.
Also, the standard web interface doesn't allow for downloading entire folders.
All the above can be done in one way or another - either by tuning/configuring Apache, adding web content management tools, or using a completely different protocol altogether (e.g. using an AFP server rather than web so users can mount the disk directly on their desktop). It's all a matter of what you want to do, coupled with the technological capabilities of both yourself and your remote users (e.g. can they handle anything more complex than 'click on this'-type directions).
