No need for windows... or an expensive mini.
If you've got an old PC laying around put freeNAS or a linux distro on it. FreeNAS basically makes the PC into a network attached storage device and is super easy to set up. Linux is a little bit harder, but once you get samba configured it's not too bad. That's what i've got at home and it works great.
If you're really set on a mac, buy an old G3 or G4 tower and use that. There's more internal space then in a mini, you can put a PCI raid card in, and the whole thing will be pretty cheap.