Then just go with debian+docker. As raid software i would recommend ZFS, its a filesystem that does both and also integrity on file level. (and lots more)
I personally would only buy new ones. No matter the brand just the best TB/€ you can get.
For MB basically every Chipset gives you 4 SATA ports. You could consider picking one that Supports unbuffered ECC memory but that is not a must. If you want to Hardware Transcode in Jellyfin, then Intel is probably your best since the dGPU with Quicksync is pretty good and well supported, otherwise i would go AMD.
For 4 drives you can use most ATX cases have no recommendations here.
exu
TheHolm