As low as I can , a couple of hundred of euros

It’s an i3 6th generation with 16gb ram. I use an ssd adaptor on my x16 pci because the m.2 is not recognizing the ssd.i do not know how to fit multiple hdd on it to get a good redundency

I have a 512 gb ssd in the optiplex and 1 tb hdd on sata. Should I upgrade the hdd to a 4 tb and use the optiplex for media server and nas? Using truenas?