I did a similar thing a bit recently and here are a few tips..
First of all, don't get too excited and expect to be 'blown away' by the Pi's performance as a "torrent server" (for downloading). It's honestly not that great. It normally does 2-2.5MB/s max while the desktop can easily get 30MB/s+ on average (I used Ubuntu ISO torrents for benchmarks). From what I gather from the web, the ethernet port (because it's sharing the USB bus - which you're also using for storage) is the bottleneck. Copying files from the Pi (via SMB) caps at 4.5MB/s and writing to it is at 3MB/s. Other people managed to make it go faster (via overclock, etc), but that's the fastest I managed to make it go.
The slow download speed doesn't make it such a nice "torrent server" for downloading. Despite having a Pi set up for torrents, I still find myself using the desktop to download because it can download stuff for a fraction of the time it takes the Pi. And sure Pi is greener, but time is gold. As an always-on seeding box though, it performs remarkably well.
For the hub, I recommend the D-Link DUB-H7
. I use that in my setup and it can power 2 Pi's, a 2.5" external drive (WD Passport), and still have enough juice to spare. I have this USB power meter thing
so I actually tested this. Each Pi takes 0.8amps, the HDD takes 0.3 = 1.9A on a hub that can supply 2.4A. The Pi's are connected to the 2 'fast charging' ports. The fact that it's a USB2 hub while the rest of the world has already moved on to USB3 means that you can get it cheaper now.