It's pretty secure, and with Knox on top of that and Private Mode on board, it's a pretty secure phone. Yes, it will be updated to Nougat (eventually), but no official date yet, just Samsung's confirmation that's on the update list. The patches are pretty quick to release, BUT if you own a carrier-branded Note, that will depend on the carrier. But within the month it's a safe bet for most.
Here's AC's take on 'secure' phones if that's your main focus
Most Secure Android Phone | Android Central