either WD or buffalo should work, but whichever is more reliable i have no idea, usually i just slap those HDDs on my torrent box and voala, heh.
NAS works as an independent unit outside of any PC, meaning they'll work even if you don't have a PC to use it with. their main use is sort of like an online storage but in this case you're making it run directly onto your network.
think of it as your PC's HDD being shared openly on your network, thats whats a NAS is.
edit: i vote for custom self-built NAS tho, you'd be able to manage the parts being used, and should be much more reliable to boot. which OS tho i don't know, but teamviewer should serve well as a remote control.
edit2: btw, if you're using windows7 you can hotlink your NAS like its a native HDD via network location, simply right click my computer and click "add network location" and follow the wizard.