First make a decision on Operating System that you want to support, will it be one of the Unix flavors (Linux/Solaris/OpenBSD/FreeBSD/MacOSX) or Windows. Does it have to be ftp or can it be a webdav server? My personal preference is webdav server:
You can map the webdav storage as a drive. Under windows you can use the network neighborhood to connect to it and I believe MacOSX has built-in client support for webdav. Doing that will allow you to treat native tools to copy files over or update those files on the fly in one location.
The biggest concern for me would be who to authenticate clients. Drop me a pm if you would like to discuss further.