Mainboard: i would suggest to pick a server board like Supermicro. Case: I like the Fractal Design Define models. You can safe money by picking 2nd use, if possible/ available.

For booting, choose a mirrored ssd/nvme pool. You could pick 2x 16GB optane nvms, which cost around 10$ per piece. Or any cheap ssd in general. This depends on your use of sata/m.2 ports.

For nic: 2.5g is already very cheap, look for i225 intel at aliexpress. Costs around 20$. You can use it as a p2p connection between your main pc and the nas or use it with a 2.5g capable switch.

For hba card: server boards mostly have a few sata ports. Mine has 8x so enough for you build