Redundancy is the most important thing in a file server/important storage on a PC, learned the same lesson the hard way

Regarding RAID5, you can set one with 3+ hdds, but the failure tolerance will always be 1 drive, different with RAID6 which allows up to 2 drives failing, as well as other mixed RAID configs, also you can use a RAID array with ZFS which allows you to correct data corruption on the fly.

The easiest way to set a file server, imo, is using a NAS OS like FreeNAS or something like that, if you like to play with things, you can later add a PFSense box to manage the network traffic and do a lot of other things