I second nas4free and ZFS is a very resilient file system too.
It’s not only cheaper and more satisfying making your own server but it’s more flexible. Also if there is a failure of any component it’ll be easier to replace than an out of the box NAS. I love the fact I can remotely turn it on with my phone which I guess you can with out of the box solutions but it can easily be taken for granted when the NAS is far out of reach, like in another country!
The HDDs are the most expensive part of any nas though. Got 6 x 4TBs plus one 120GB ssd for the OS which needs nothing like that capacity but I had the ssd lying around dormant. I did buy a dedicated case, psu, mobo and one extra sata card but you could decrease the cost by getting an old computer and chucking in some better value hdds. Obviously the largest capacity hdds will cost more but you may not need that much. Bear in mind you’ll need to consider redundancy in case of hdd failure so make sure your case can hold the number of hdds you need to install.