hard drive - File Storage NAS - Do i need to replicate in addition to RAID 5

I am going to buy a NAS server soon. I am pretty sure I know the one I am going to get.

It can hold 4 hard drives.

If I RAID 5 then I will have about 8gigs of storage.

I understand if there is mechanical failure with a drive - the RAID can re-build a new drive that is inserted.

But...

For added peace of mind, should I be replicating everything to another NAS or
RAIDING 2 drives and replicating on to 2 drives.

I am not a server specialist. Just someone who wants security on a small budget.