FreeNAS is aimed at NAS purpose, and additionally at rock-solid and reliable hardware to protect your data.
And the truth is, there are chips that are supported and proven to work under FreeNAS (and the underlying FeeBSD), others that don't work at all, and others that might work, but not as stable as you'd want.
So the recommendation is to stick with the supported and proven-good stuff like Intel NICs, Broadcom SAS and so on.
And in general, server-grade hardware is recommended because it is aimed at professionals and professional use, and is built to be stable and last longer.
Of course you could build up just nearly anything, install some NAS distribution and use it as a NAS, and it will most likely work, even Raspberry Pi can be used as such somehow.
But that way, it is more likely that something might happen someday, and your data is gone, or even worse, silently corrupted.
FreeNAS users usually want their data to be as safe as possible, and a stable system that can and will run untouched for years.
So they go for professional equipment, and often buy used components.
And here are many people that know what they are talking about, because they earn their money with server administration and such.
The answers you got here may sound a bit harsh, but they are true.
There is hardware that is well-supported and proven to work, and you should stick to that for a new build.
You would't buy a new Maserati with a trailer coupling to pull your trailer, you would buy a truck instead.
Repurposing already owned stuff is another thing, an old gaming rig or something found in the depths of an attic is nice to try if FreeNAS is for you, but almost always it is better to sell that stuff and get appropriate parts instead.
Here some more detailed explanations if your selected part are ok, and if not, why they are not ok:
5 WD Red 2 Tb
3 WD Black Enterprise 2 Tb
You can use your 8x 2TB disks for FreeNAS, of course.
The Blacks may be a bit louder and run hotter than the Reds, and maybe 24/7 on-time will shorten their lifetime, but you could replace them later.
1 Samsung EVO 850 500 Gb SSD
If need: 1 Samsung 840 250 Gb SSD
Your SSDs are actually too big for FreeNAS, it only uses about 16GB (and you cannot put anything else on that disk by default), so the rest of the space would be wasted. But if you already own it and have nothing else to use it for, you can use it for sure.
The recommendation is to use two mirrored 16GB+ USB sticks, or (better) a cheap used (server-grade) SSD like for example Intel 320 or S3500 DC with 40 or 80GB.
ASUS TUF B450M-PLUS GAMING
The mainboard you picked is aimed at gaming market. Many of its advertized features like RGB illumination are absolutely useless for a server, and onboard sound is genrally not supported at all in FreeNAS. So you waste your money on totally unnecessary things for a server.
And you get components that are not well-supported (like Realtek LAN) and might result into problems later.
Corsair 16GB (2x8GB) DDR4 3200Mhz CL16 Vengeance
For the RAM, you should go for ECC to protect your data. Storage goes bigger and bigger, and so bit errors are more likely to happen. The data on disks is secured by ZFS, but if it gets transferred wrong from RAM to disk, you need additional protection there, so ECC.
And the additional cost is not so high either.
That PSU is overpowered, but of course you can use it if you already own it.
The Delock SATA card could possibly work, but it could also not work at all, or (even worse) give you strange errors sometimes, I don't know.
So instead, you should go for a SAS HBA card (with Broadcom SAS2008 chip for example), this allows you to connect 8 drives directly, or by using expanders, even 24 or more drives.
And those are proven-good and rock-stable, as these are used in thousands of datacenters.
They are quite expensive if bought new, but used ones are fairly cheap.
In general, Intel is supported best, but there have been successful AMD builds as well.
So the mainboard could for example be a "Supermicro" or "ASRock Rack" board with ECC support, a supported CPU that supports ECC (Celeron, i3 or Xeon), and ECC RAM.
Server boards are ugly green instead of the sleek black gaming boards with fancy heatsinks and disco lighting, but they offer really useful features instead.
Like for example a feature called IPMI, with that you can access your server via browser, you don't ever need to hook up a keyboard, mouse or monitor to your server, you can even do BIOS settings with it, this is some really useful stuff.
What exactly did the guys from the other forums advise you?
You can get really good advice for really good combinations here, if you listen to the people with a vast amount of knowledge around here.
But you have to be willing to listen to them, they make their suggestions not to offend you, but to help you, free of charge.
