Try the QNAP NAS boxes
I have had two of them, one a single-drive version and now I have a dual-drive version.
They have the ability to do all that you require, including a media server and a BitTorrent client. It has a Gigabit Ethernet connection, and USB ports at the front and back.
If you get the single-drive version then when you want to add another one, you can just buy a second NAS box and RAID them across the LAN. This is not something that I have tried myself but QNAP claims that this is feasible. There are updates available on its website from time to time which add new functions or fix some bugs. It runs on Linux so is open and extensible.
I'm very happy with mine, although I use it mainly as a back-up device to my PCs connected to the LAN.