On some routers you can plug in a usb device and have that accessible on the network. However, since you are backing up multiple PCs and storing a bunch of media files, I think a NAS would be a better option since it can offer redundancy and increased capacity. A NAS and home server are pretty much the same thing, just designed for different use cases. A NAS doesn't need very powerful hardware since it is just transfering files. A home server might need some more powerful hardware since it will usually be used to run game servers and web services, among other things.