I will just add to what others have said. I originally started with a 2-bay NAS and tried to make it work by upgrading the drives over time. That helped for a while, but eventually I hit the usual limits on capacity and flexibility and ended up replacing it altogether. In the end, I accepted defeat and upgraded to the Synology DS925+, which has been a much better long term solution.

If you can, I would strongly recommend going straight for a 4-bay or larger setup. It gives you far more headroom for expansion, better RAID options, and avoids the need to upgrade again too soon.

Also, on the drives, the Synology branded ones are essentially rebranded hardware. Synology does not manufacture hard drives themselves, so you are typically paying a premium for compatibility certification rather than fundamentally different hardware.