lol you guys are funny. I priced out a simple i5 box, onboard video with a 1tb 850evo. Simple. It will be fine. They wanted a raid 1 setup, but I think a single SSD will be fine. Relax, we got Carbonite bitmches.
Say you back it up once a week, and sometimes people forget to do it. Then at some point the SSD fails. Your latest backup is several weeks old, and you don't have a spare disk available. That means you quickly have to get a replacement from somewhere and you need to hope that people have the recent data on their local computers too still.
If you program your own software it's a pain and you may have to do a lot again in such a situation, if you work on project basis it means the project just got set back significantly and you may be losing out on money as you won't be able to make the deadline.
A NAS is a good idea if you know you can get replacement hardware for it if needed (that's a large part of a support contract, that if it breaks down you can not only get a replacement harddisk but also an identical RAID controller or so that allows you to get back up as quickly as possible). A RAID setup (not RAID 0) means that a broken harddisk doesn't immediately put you out of business, while a UPS means that there's no data loss in case of a power failure or power surge.
Also consider whether or not you expect or plan on growing as a company. If you get bigger you may want more functionality, you may want an actual fileserver, allow remote logins, have a firewall or other functions. If that may be soon then it's worthwhile to consider getting a small server of a big brand (so you know spare parts will be available for quite some time). That way you can customize it for the future, and for example also put a backup drive in it already.