With the priority to have a simple, safe (raid) and low power solution
I suggest an excito model with 2 internal bays for 2.5" HD.
Actually the model B3 should have the space for such small HDs.
This would be a B4 2slim
As an "open" device the user should be able to add the HD's by himself.
The booting and firmware running from a SSD or a fast-flash would indeed be a nice to have too.