What I would personally do is get a standard pc, anything that you can plug sata hard drives into and get for cheap, get two hard drives of whatever size you deem necessary, and then put them in raid 1, this means that both drives are a copy of eachother, so if one dies the data is okay.

Afterwards you can put whatever software you want, but I would need more info on what you want to do with it to give recommendations there. ie: general file storage only, or also stuff like online document editing? Backups from what kind of system? Only macs or something else too? How technical are you for setup and stuff?