FYI... These units are really great. I have been using them since I got my DS212+ in 2012. Not 1 issue for 7 years of owning it. However, I did upgrade to larger drives a few years ago. My point is that what you buy will probably last a long time and they have many more uses then just the Channels DVR. I never used the DS212+ for Channels since I had other options. It was primary my offsite backup for the office DS. But now I use the DS1019+ for the same offsite backup and also for Channels DVR (internal and external access) and Plex Movies (use a lot for internal and external access). I also use a DS918+ for local backup and for testing things like the Virtual Machine Manager.
So my point is look a the cost of all of these units and compare cost and maybe depending on your budget spend a bit more. Also, compare disk drive cost and maybe buy a bit more than you need. I use the WD RED NAS drives (5400 speed). I also added the SSD Cache (on the bottom of 1019 and 918) but not sure it gives that much of an improvement but felt why not with with the slower drives. If you got one of these you could always add later.