As others have said, just stay away from the used server stuff. It’s cool to look at and run and setup, but it’s not cool to pay for the power.
There are use cases for both the systems you listed @Sedanxio. I have found that Plex likes more cores, so the i3 based system may struggle and the a8 system may be better. If all you need is a place to store files and backups, then the i3 system would be better.
Don’t fall in the trap of used server hardware like I did (and probably quite a few others here). I ran a massive dual processor system for about a year before realizing it was way, way overkill. Downsized to a much more modest Xeon E3-1275L v3 based system with a consumer Asus motherboard and have been golden ever since.
I realized I forgot to mention that my current NAS has a 4 drives backplane only (chassis is a Inter-Tech 2404s). It’s a server based on an ASrock EC224-D2I mini ITX with a pentium 3420 and 8gb ecc ram and it has 4x 3Tb Seagate Ironwolf in RAID 6 which is 80% full.
I do a lot of picture editing, with the files directly on the NAS, computers backup and my wife uses it for storage only.
I plan on setting up a plex server on the new one to digitalize our whole media library.
As @marasm suggested, I will go for the a8 9600. I already have 2x 16Gb ECC DDR4 for it (which works in ECC mode with this CPU and motherboard combo).
@Lauritzen, the EPYC I mentioned is the AMD equivalent of the ATOM C3558 whiche are low power parts. I would love to have one like this but it would be far out of budget at this time, not mentioning they’re nowhere to be found yet.
10Gb is also needed as I work on my files direcly from the NAS.
So far the setup would be:
Fantec 2080x07 2U chassis with 8 drive bays ( I have a 19" 33U rack at home to house my networking equipment and my other server for remote desktop)
Seasonic SS-500L2U that will arrive sometimes next week (I will mod it with a Noctua 60mm silent fan)
AMD A8 9600
ASRock A320m PRO4
2x16 Gb Crucial DDR4 2133 ECC
6x 3Tb in RAID 6
Intel af-da 10Gb NIC
Dell PERC H310 in IT mode
I’ll probably use Open Media Vault to do this, as my actual NAS but would you suggest to use FreeNAS instead? I know it has more advanced plug-ins and that I can setup some cache drives with it but I’ve got far more experience with OMV.