https://www.freedombox.org/apps/
Whatever you end up doing immediately after you finish setting it up, throw some files on it you don’t care about and practice breaking and reassembling your RAID a few times before you put anything important on it. You want to understand the basic process before things fail.
I successfully ran a modded java minecraft server for me and my friends for years on an ancient thinkstation with a xeon E5430 (quad core 2.6 ghz w/ddr3 ram) doing double duty as a NAS. That old xeon couldn’t carry your i5’s jock on single core performance, which is your main concern. As long as you’re not running huge kitchen sink packs with giant complex bases I think you’ll be fine.
OK democratic leadership, you win; I’ll quit voting for your party.