RAID questions with ML
My initial reaction from how you staged your question: building a NAS from the ground up is a complex set of interrelated dependencies in both hardware, software and connectivity. You might be better off purchasing a ready-made or self-contained NAS unit.

You need to ask what you expect from a NAS: lots of storage or redundancy. The two-drive RAID you mention could be configured a couple of ways, and you may not be able to read the volume without the original RAID controller from the NAS.

Your inquiry into starting with one drive then adding additional drives sounds like some JBOD (just a bunch of disks) setups. JBOD only offers los of storage, but no redundancy.

RAID and NAS can both be elements of a storage solution, but in and of themselves, the two acronyms can also spell HEADACHE.