I'd look at a review like this one and choose from one of them:
https://www.youtube.com/watch?v=ga-jBSo3Nbc
Some things I think are really important:
1. You want to have your NAS on an UPS to handle both spikes and dips
2. You need ECC RAM. This should not be optional in your mind.
3. I strongly, strongly recommend you use ZFS, which is why I use TrueNAS. ZFS has many features you want, but the big ones are that its ZFS RAID is built right into the filesystem, and more importantly, scrubbing. I think scrubbing is a crucial feature at this point.
4. You could build it yourself in terms of both hardware and software, but I think that there are enough details in both that you really do want someone else to do it for you unless you're familiar with these issues.