If you want cheap, then turning an old PC into a dedicated Linux box would be the cheapest.
But if you want always updated, sophisticated solution, Synology rocks. I've been running Synoloigy drives for about 10 years or so now, ever since Windows Home Server was retired. Initially used 2-drive solution, now up to 4 drive model, love it. I never have to worry about making room for anything. Backs up 3 PC's and a Mac, and has our family documents and photos and videos on it. And Synology has a virus checker, firewall, and continually updates their security features. So I feel reasonably secure accessing my files remotely. I use Western Digital Red drives made for NAS use.
