A DS918+ is a pretty solid choice. When considering a NAS, always gets more drives than you think you'll need. And when you start buying hard drives, get them bigger than you need. Because your data storage needs will never decrease, only grow.
Recall that Bill Gates famously said that no one would ever need for than 640KB of RAM.
When built my NAS, Western Digital Red drives were pretty expensive. And in looking on Amazon right now, they are again. However, you can just buy the Western Digital Easystore external drives. There are plenty of guides to shuck the case off of them and you're left with a Western Digital Red. Right now, Amazon has 6TB Reds for $231 and 12 TB for $344. Ouch.
But they also have Easystores for $144 for 8TB and $250 for 12 TB.
At the time, I bought seven Easystore 8TB drives and have them in a FreeNAS array. If you like playing with computers and tinkering, FreeNAS is the way to go. If you just wanna plug in the drives and follow a wizard, get a 918+ and put in some drives and you're golden.