Buildapc

I'm interested in setting up something to act as a file server. Think of it as "the cloud" but local. I've never built (or bought) something specifically for this, so it's a big foreign to me.

I think really all I would want is something that can store a lot of TB of data easily. It doesn't need to be fast. It doesn't need to be able to stream media anywhere. It really only needs to be able to act as an SFTP server, maybe run sync thing (new to that), and maybe act as a NAS. My gut feeling is something like 10+ TB might be a good amount to start. Something that won't fill up quick and that I can put big things in (like a full system image of another computer) without concern.

What would be a good way to go about this? Building a computer like normal but getting very cheap stuff? Getting something pre built or used (like surplus office stuff)? I'm just not really sure where to begin.