Expanding storage and back-up

[Alceister]

I'm looking to expand and optimize data storage of media and documents for my PC and household. Presently, I utilize a mixture of different HDDs (in addition to a SSD system drive), which are as follows:

• 
  ■ 2x 3.5" WD Blue 1 TB HDD and 1x 3.5" WD Blue 160 GB HDD. These are slotted in and out of a single 5.25" hot-swap tray as needed. I keep no permanently-running internal drives except for my system drive, mainly because it isn't necessary to access them at all times and I am a stickler for noise and power efficiency. It used to be that one was used for storage and the other was used for system back-up, but now I'm starting to get full on the one I use for storage. The third is used to provide back-ups of important documents.
  
  ■ 1x 2.5" Seagate 300 GB HDD mounted in an enclosure. This I mainly use to store videos and provide expanded storage to an Android TV box.

I am planning to expand my storage, but three questions come to mind:

First, is it worth getting an NAS HDD if I am not planning to have it continuously running or providing a permanent internal mounting?

Second, should I consider a second SSD to be used as back-up or to provide redundancy instead of an HDD?

Third, is there any merit to my existing layout or should I consider moving everything inside?

 

[CountMike]

Backup is best when used as offline and stored in safe place. For rest, it's fastest if installed inside in SATA ports. OS and programs on SSD(s), the rest doesn't matter.