Casings are all generic no name stuff. eBay would have them cheap. As for hard disks, I always have had good experiences with Seagate. Make sure to get the 8Mb cache model.
Does it need to be USB, I recently went for a 300GB NAS from Buffalo and am very impressed with it so far: http://www.ebuyer.com/UK/product/75274 (I wish ebuyer had an affiliate program).
No I have it plugged in to my network, you can get wireless ones though or you could use a cross over cable.