Your basic components will be an old desktop you have lying around and two hard drives. Put the two hard drives in RAID 1 (mirroring) set with either a network share and/or FTP access to add/remove stuff from the array. The drives optimally should be the same size, but if they aren't that is OK, the amount of redundant space available will the the size of the smaller of the two drives.

Depending on what you have lying around this might not cost you anything. However, if you are going to spend money anywhere it should be on the drives themselves. You probably don't need anything fancy, just a pair of 5400RPM HDDs that are large enough to hold your data, plus some room to grow.

You can use any OS of your choosing as basically everything supports the requirements. Linux, Windows, and TrueNAS come to mind as viable options. You may or may not want a third, tiny, drive just to boot the OS, particularly for Windows, as it can make things easier. I personally use Linux for my basic NAS with SFTP access.