External HD serve media through iMac or do I need standalone media server?
Can I buy an extra external HD and use it to serve the libraries to the other devices through my main iMac?


In the case of your AppleTV, it see's content that's in your iTunes Library. So as long as your iTunes library is on your external drive, your AppleTV will have access to it. The method for moving your iTunes library to another drive has been well covered on this site. I suggest you check out:

https://apple.stackexchange.com/a/31393/292

There are some things to consider when moving your iTunes library to an external drive:


External drive access can be slower, especially if it's over an already busy USB buss, so take that in to consideration if you need to stream multiple high definition videos out of the box at the same time. You might want to consider a FireWire or a Thunderbolt equipped drive to ensure you've got sustained throughput rates to deal with your streaming needs.
Time Machine doesn't back up external drives. So your iTunes Library won't be getting the Time Machine treatment. You'll need to back it up yourself with something like Carbon Copy Cloner or a cloud backup service if you want to make sure your library data is safe.