There's a radiator, and it needs a fan for cooling, or else you're in big problem.
Pico-ITX is to avoid. Slow, VERY slow, and it lingers around $250, the board and CPU.
So, if you need to build one from scratch, here's two possibilities:
1- Build a Slot 1-based machine with AT LEAST 256MB RAM, and a big HDD, and remove all fans. (P2 233-350 is enough)
2- Get this Router/TorrentBox:
It's $234, but it's a wireless router, with an intergrated Torrent program and a 160GB HDD, that you can upgrade. It's great, silent, and very small (The size of a 3.5" HDD.