For the build, you seem to be thinking as if it was a PC build. For instance a firewall doesn't need all that storage. In fact NVMe is totally overkill. SSD is perfect. HD is good but not as much.All you need is a high clock CPU,especially if you are on PPoE from your mention of a modem.Forget about wifi on a firewall based on OPN. It's based on freeBSD that is not great for hardware support for wifi cards. There are some supported and OPN can use some of them but save yourself the trouble. There WILL be troubles to overcome.You don't need a GPU.What you need is a small device with at least 2 network interfaces, preferable Intel, not Realtek. A CPU of at least 2 GHz, 2 cores will work, 4 are better. More are not strictly needed. One SSD of at least 60 GB or more. 120 is more usable. More is OK but unnecessary unless you plan on logging a lot.Do you have PPoE from your ISP? Do you have a switch already?Finally. The ability to stream is depending on your what throughput you can achieve on the WAN interface from your ISP, what is your current bandwith? Together with the PPoE question, it should give enough info.The CPU family (Celeron D, Core, AMD Ryzen, etc) is of no importance. As long as is AMD64 it is fine. What matters is the single core score.Have a look in the docs as well https://docs.opnsense.org/manual/hardware.html
So here's the thing about WiFi cards: They're designed to be clients, not Access Points. You will have a very bad, but memorable experience if you try to use them as Access Points. Buy a purpose built AP, like an Omada 620. A router and a NAS should be 2 completely separate appliances, and shouldn't be combined. You don't haul lumber in a Prius, don't store data on your router.