I used an old NUC with ubuntu to do this, except the backup was performed by my server rather than software on the NUC, makes it with easier that way, imo. Auto start in bios and Wireshark set to always connect.
I do recommend a wired connected if possible. So if someone messes with the wifi it won't matter.
No, I don't keep track of consumer NAS as I DIY everything. I can recommend that but I recognize it's not for everyone. I see that it's popular to use an Intel NUC or similar as the server hosting Plex and other apps, and leave the NAS simply for storage; that's very enticing.
Do not put a rack mount server in a living room, especially a 1U. The boise will never be manageable for that kind of room.
Also, the server will idle around 100 watts. The NAS much less.
Idk about AUS specifically, but you can generally get a 5TB external spinner for $90-$100 that you can Velcro to the top of your NUC. Minimal size and works great, runs direct off USB power. No way would I pay for a 8TB SSD in there.
4TB SSD bottom out around $150 (US again) and would be perfectly good for Plex (don't need quality for that), but again it's a cost issue.
I'd also consider something larger than a 512gb m.2; that's a lot of wasted space and 2TB-4TB m.2 is just as cheap as SSD.
Firstly, there's no reason your plan won't work. So you can go forth and conquer if necessary.
I'm not a fan of raid 5 unless you can articulate a need for a function that it uniquely provides. I would especially warn against it when used in conjunction with a USB enclosure. Lot of related horror stories on this sub, and I have one of my own. I'm a big advocate for unraid and other solutions that provide parity without striping data; it's especially resilient in a homelab environment where tinkering is typical and mistakes are expected.
I'm also not a fan of a local backup. Whenever you have a single system hosting the live and backup data, you subject that data to shared threats. Sure you can recover from more data loss or disk failure easier, but other threats have the potential of destroying your backup along with the live data.
Why retire the 804? It's still one of the better cases out there. If it's in good shape and has the parts, it'll go for an easy $80-$100 on Facebook.