Why is the hardware not recommended? Is it because its missing ECC memory?
ECC is just one instance of the general theme "consumer/gamer PC repurposed as NAS". ECC would be better, but its lack is not a fatal flaw and you undertand the issue.
Generally, you're paying for features a NAS does not need (audio, USB 3.2 2x2, PCIe 4.0) or barely needs (video output, just for setup) but do not get enough SATA ports to fill up your case. Again, just look up the Supermicro X11SSH-F: 8 SATA, 2*PCIe x8 for expansion cards, VGA output from the BMC for setup even with a "F" CPU—and that's just a close server counterpart to a consumer Z390 mATX motherboard.
I feel no sarcasm in
@ThreeDee's post, which nicely made up my points about old server hardware and DDR3 RDIMM being super, super cheap with a practical example. He likely also made up my other point that you're not familiar at all with his part list…
I suppose you don't know server-grade hardware (neither did I before I began thinking about FreeNAS) and thus picked up parts from the consumer hardware you're familiar with. I understand from an earlier thread by
@SeaWolfX that new server hardware is rather expensive in Scandinavia (Norway in his case). The second-hand European market is not as favourable as the US market. But at least Sweden is in the EU, so you could buy from eBay.de and not pay crazy high taxes on import, right?
If you can send back some or all of your order, we may discuss your options to get cheap server hardware and try to help you save money yet get a better NAS. But you'd need to expose what you want to achieve (just store? / also VM/jails? network?) and what are your constraints (noise? / keep the microATX case? / etc.).