Thanks to CFU for the summarized contents, will try to contribute. Agreed DIY means learning the complications, which is better than depending on adverts and general here say, for ready made NAS.

Sharing from my experience ya

First rule of NAS

1. Secure your data
2. Back up your data
3. Take care of your data

Second rule of NAS

1. Choose OS wisely
2. Choose hardware wisely
3. Have back up hardware that is repeatable

Third rule of NAS

1. Contribute to community
2. Expand network relevant to the chosen OS
3. Pool resources

There are 2 parts to NAS-Data & Services