A NAS is really just a bucket to put stuff in. The software that puts (whatever) in the bucket is what matters. It sounds like you need some sort of backup solution, plus a place to put those backups.
I have two PCs relevant to this discussion - my desktop computer with all my super-critical files (photos I don't want to lose, etc) and my home theatre PC with a bunch of giant hard drives in it to store TV, movies, blah blah. The desktop runs Windows 8.1, the HTPC runs Windows 7. They are both on the same network.
I am using CrashPlan, backup software I like quite a bit [note: they are not paying me to say good things about it right now]. CrashPlan is installed on both machines. CrashPlan is set up to:
1. Back up Desktop to "Cloud". This sends EVERYTHING I think is important (NOT a system image, I have to reinstall if something blows up) to CrashPlan itself online. This costs me some money every year.
2. Back up Desktop to HTPC over my LAN. This is the same backup as 1, but uses space on my HTPC. I now have 3 copies of my critical stuff - two at home, and one at CrashPlan.
3. Back up critical HTPC stuff to Desktop.
#2 and #3 are free, #1 costs like $99 a year for unlimited storage.
As you can imagine, the home theatre PC doesn't have to be a home theatre PC, it can be a "backup" PC that ONLY runs CrashPlan on a regular version of Windows that is otherwise sitting idle. If you upgraded your WHS box to Windows 8.1, you can use a feature called Storage Pools to smoosh all your hard drives into one drive letter and not worry about where the data is going in particular. You could also do roughly the same thing with a NAS - just tell CrashPlan or [insert other backup software here] to put the files on the NAS network address, but since you already have a computer, I suggest just using that.
If you want full-PC images that you can instantly restore to, you could use Acronis TrueImage, I've had good luck with the corporate version of that here at work (Acronis Backup and Recovery). I haven't used TrueImage in a while though. It's like what Norton Ghost used to be. The newer versions of Ghost sort of stink in my experience. You could also use a combination of free CrashPlan and paid TrueImage, or whatever, just be careful not to get backups in your backups and make all your files gigantic!