What is the best NAS in your opinion, 4 or 6 bay, 8-16GB, I have just purchased 4 WD 4TB RED NAS drives and so would like to buy a GOOD (Recommended by you) NAS BOX, Please let me know.
It is always recommended to have a few spare bays. This allows you to expand your RAID later on without a need for a new NAS or expansion unit.
You can start with 2 or 3 drives at the start. You can also use spare bays for a "hot spare" drive which means that if there is a drive failure in a RAID it will be replaced automatically with this hard drive (no time wasted ordering a new one).
Qnap 53D series is a good choice offering multifunctionality.
I hope this helps.