To do this, I'll use Samba to share the files over my network (a mixture of Linux and Windows clients), and MiniDLNA to manage media serving. MiniDLNA will allow any UPnP compatible device on your network, such as a television or games console, to access the shares on your Pi. qBittorent will be used as the torrent client. Fsniper will be used for auto-download feature..
I also use fsniper to automatically sort all locally downloaded content into the appropriate files on the NAS.
As an alternative to this, you could use an operating system like FreeNAS or XMBC. I prefer to keep Raspbian and bolt on the extra bits I need.
This guide assumes you are using the 06-12-12 of Raspbian. It should also work on the latest version, but I can't guarantee that as I've not had chance to test it yet. I used the following IP scheme:
Pi Ethernet Port: 192.168.1.10/24
Pi wlan0: 10.0.0.1/24
Note: external hard drives on the Pi should be connected via a powered hub.