A HP Microserver N40L is a pretty decent NAS. If you're willing to tinker a bit, you can also add some additional harddrives outside of the coldswappable drive-shafts.
It's an excellent case design - there's a huge fan at the back pulling air over the 4 drive bays and my TB drives never go over about 25C. It's in a cupboard and unless I open the door it's totally silent.
From what I've seen of other NAS-style devices, it's pretty much unbeatable for value since it does all the same stuff but is ultimately a real, if basic, server.