I don't consider myself a Linux expert, I have enough knowledge to get by. I have had no problems setting up my Sheevaplug to do pretty much exactly as you describe your requirement. It uses twonkymedia to serve media from my USB HDD, it's got a webserver, and wTorrent (rTorrent with a web interface), and a few other tricks up it's sleeve. Running mine with the alpha6 installer, updated to the latest jaunty with apt-get dist-upgrade, and get the latest pre-built kernel from
http://sheeva.with-linux.com/sheeva/If you have a little Linux knowledge, I say jump in, there are plenty of people on these forums, the general ubuntu support forums (although most scratch their head with ARM/embedded questions and don't quite get why you don't want full X-Windows with your apt-get...)