Have a look at the ASROCK N100DC ITX. There are a few NAS tutorial/build videos with it on the internet.
I can totally see it having the system on a cheapo M.2 drive and your 2 HDDS connected to the onboard SATA ports.
A few things I would be wary about:
Does the integrated HDD power supply have enough juice to spin up your particular drives?
Do you expect to want more drives in the future?
People tend to match that board with a chinesium M.2 to SATA (x6, x8 port) adapters, which are heavily frowned upon around here. I have one such adapter on order, to check it out, but unless you find credible reports of it being 100% reliable, I would not trust any critical data to it. Bear in mind, you will probably need a separate power supply for extra HDDs, besides the laptop brick you would need for the system itself.