I can only recommend the one I have myself and it is a Synology (4 series witch means with 4 bays = 4 slots). Anyway, when looking for a NAS, it is not only the price but also the Hardware and Software.
With some research, I came to choose synology as per great reviews for its stability, updates, and being silent!
I suggest this one hardly. I am 100% satisfied with it. Very powerful, lots of apps you can download, easy to set up etc.
But the best is Synoload (may be a different name for IOS, as I have Android).
With the torrent already integrated in Synology, you can from you tablett connect directly to the NAS torrent download section, and get stuff without doing anything really, Just a few clicks and here we go.
For backups, you can choose different options, raid 1-10.
set up your own email server
wordpress server etc etc
Anyway good luck with your choice.