Bob wants to buy a Synology NAS and discovered that it doesn't come with hard drives. Leo says that's correct. That way he can put in the right hard drive for his needs. If he's going to stream a lot of video, he'll want a faster hard drive. It isn't a cheap NAS.
To help with costs, he could try a two drive enclosure, rather than a four or five drive model. The software is the same, and now it costs half as much. Leo recommends the Western Digital RED drives. He'll also want to use a good media server. He should keep in mind that he'll want to use redundancy, which could take up a third or half of his hard drives. Can he then get a larger drive later? Leo says yes, it'll rebuild automatically. But he could easily get it for under $500 to start out.