(use NAS)...to make sure I don't lose them due to to a hard drive failure.
Even a RAID format won't give you the level of protection you probably want. Equipment can be stolen, kicked over by accident, damaged in a fire or windstorm, wiped out by an electrical surge, and so on.
A RAID only helps with one particular type of loss (single drive failure in a correctly configured multiple drive setup.) It doesn't do squat for the other perils.
I recommend a second, backup drive that spends most of its time unplugged and put in a safe spot. (The safest is off-premises storage when a fire, etc. won't get it.)
I just bought another 750 GB USB drive for backup purposes. $170 was a pretty small investment for the time I have invested in assembling a collection similar in size to yours (excess of 2000 albums.)