Re: What have I missed?
"...IP Camera function..."
As far as I'm aware, the CCTV camera free software is good for one camera - more and you need to pay. You'll need to read up on it, as there is some discussion that as a camera suite option, there are better choices out there. Sure this one lives on existing hardware - but that's something you need to evaluate if it's critical to your needs.
"...WD Green edition..."
Not suitable at all for an always-on home/office environment. It auto-parks the heads periodically, which makes sense on a desktop, but in a NAS environment it'll kill the drive fairly quickly. The WD Red is a better option - it's basically a Green with modified firmware to not park the heads so it doesn't self destruct as quickly. Yes, you're paying a premium for an option that USED TO BE FREE, but they have to make their money somehow. The WD Enterprise series are even better suited, have a better MTBF, and faster, but the cost is probably not justifed for this lower end NAS.
This suggestion is cheaper, (I have one as well), and most certainly justified with cheap drives as it only gets used intermittantly so I can travel with bulk data. However, it only works with up to 3Tb drives, the DNS-320L can deal with 4Tb drives, and you're heavily limited on "applications" if that's important to you. The newer Synology DS214 in this article can also deal with 4Tb drives. Do your homework and see if your small NAS can deal with 4Tb drives, as it would not be worthwhile going for a "cheap" sellout early revision if you need to upgrade later.