Small office backups, small NAS (RAID) vs single disk

Solution 1:

• Adding a disk to a server is not a backup. Period.
• Separate backup disk: If your main system is down, do you want to learn that the all-important file on the backup disk is unreadable?
• Mini NAS: The best solution in your list

but:

You should want to have an offsite backup. The NAS will also be destroyed if your equipment is destroyed in a flood, fire or thunderstorm (or stolen).

Solution 2:

You should follow the 3-2-1 rule for backups.

3 copies on 2 different types of media and at least one of those is off site.

I'd suggest the NAS, and periodically copy your NAS to DVDs/tape to take offsite, or duplicate the NAS to an offsite NAS over the internet.

No matter how small your business is, the total loss of all of your data would be a catastrophic event, so keep offsite backups. Those offsite backups could be in "the cloud" but they definitely should exist.