I don't know Amahi home server, their "Greyhole" technology isn't something I have used, so I can not speak about it.
Personally I use FreeNas and have used Zentyal in the home as well the later being more an equivalent Windows SMB server than a home NAS.
FreeNAS is based on FreeBSD which is a UNIX operating system similar to the Linux Amahi and Zentyal and would be my choice if your old hardware supports it (FreeBSD is abit more restricted than Windows and Linux for the hardware it runs on). A home storage machine doesn't need to be that powerful my current FreeNAS box is a circa 2007 Asus P4 board with a q6600 cpu and 8GB of ram and 6 2TB disks of various vintages in RAIDZ2 config. The operating system runs from a 16GB flash drive that was a freebie at a conference. Power consumption is not the best but I have a 100% green energy tariff with "Good Energy" so my guilt is assuaged.
For Backups I use rsync.net to backup my FreeNas box and use the Bacula plugin to backup the Windows, Linux and Macs at home. You would need another box for a "file director" which for me is a very cheap NUC clone or take a look at the Crashplan plugin, which I haven't used.
I hope that helps, I am not sure if I am the best person to ask about home stuff as my own setup is pretty non standard and my job makes me tend towards unnecessary over engineering at home.