In iTunes Preferences you should be able to change the location of where media is stored to any accessible local or network drive.
You can also store media in several locations once downloaded by re-adding files store on other drived by reference rather than copying to teh iTunes media folder (once things are in iTunes, find them in the iTunes folder, copy to the desired destination drive, then delete from within iTunes interface itself - next re-add the files by reference. This used to work by holding alt or similar when dragging and dropping to the itunes icon or setting iTunes Preferences not to copy on import).
The issue for AppleTV is that when the internet goes down it won't be able to authorise the locally streamed media to play - also you'll need at least one computer running to stream via Home Sharing. Even if you put media on a NAS iTunes is a helper app that fetched the media being played and sends it to AppleTV. NAS itunes servers don't work for Home Sharing.