Maybe try to boot Unraid or OMV via USB. Then the software is still patched.
Depends what your after ... I used to roll my own storage servers, but had a constant fight with either something going or performance issues - I then purchased a NAS as an experiment, and never looked back.
Personally, I say look at what else is out there, you have a couple of other vendors on the market, including ASUSTOR and TerraMaster.. from experience, If you want cheap, an efficient.. look at the ARM-based models, but they'd still run rings around something 12-13 years old.
I'd say steer clear of QNAP, they've been having a bad few years with regards to vulnerabilities, the other issue this causes is almost weekly critical firmware updates, which requires a reboot (not an issue for many.. but intolerable when it's providing NFS for my ESXi and K8S hosts).
On the i7, you could give TrueNAS a try.. but I'd suggest you wouldn't want to run that long term, as it wont be anywhere near as powerful or efficient as the latest 2-3 generations.. even an entry level i3 N305 has 8 cores these days - and it'd be a PITA to get it setup, only to discover you need to replace it in 6-12 months...
Which ever way you go, see this as an investment... which I suspect, you already treat it as, especially since you've done so well from your old machines (call me impressed)
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