I’m running my NAS on a 12 year old motherboard with 16gb of ram the max the board supports. Though I wish I could bump this up now after running this system for 9 years.
I would recommend having a board with at least a PCIe slot so if you ever need more drives you can plug them all into a HBA Card. My board has 3 and I use 2 of them at the moment. One for the HBA card that supports 24 drives and another for a 10gb NIC.
The third I would probably use to add another HBA card if I expand drive quantities.