I know that thats why I make backups with a cheap WD My studio external HDD once in a while. And I sync some other files such as pictures to dropbox on top of that.You can put the NAS box anywhere in your house if you have a proper network setup. The main advantages of a NAS are:
-redundancy with RAID
-expandability
-file sharing across all your LAN (and remote) devices.
If the SSD on your macbook becomes defective, you could lose everything on it (imagine losing gigabytes worth of family photos).
With a NAS with 4 HDDs running RAID 5, you can afford to lose 1 disk at a time without losing data.
Edit: To answer the OP question, I too recommend a Synology NAS. The DSM software is simply awesome and the transfer speed is one of the best (depends which model you get). If you do choose Synology with 4-bay or more, I recommend format the drives using Synology Hybrid RAID. This way you can mix different sized HDDs with minimal performance loss. With regular RAID, you'd have to replace ALL the drives if you ever want to upgrade the capacity.
Is there not anything cheaper than synology? Can you hook up an external HDD to an apple tv and access your files using your apple tv while on your MBP