This is very controversial topic.
Yeah, some reasonably cheap consumer boards for AMD supports ECC. But that’s not my cup of tea, perhaps somebody else can help with those.
While mirrors have better IOPS and are definitely advised for the VMs and block storage, they are not really needed for the media storage. With raidz, you would have more usable space. You would need more drives from the start, though, or deal with raidz expansion and add them one by one. You can read about the (performance) benefits of each pool layout here.
Well, I think many cases would meet these requirements. Especially if you don’t mind using non-NAS standard tower case.
And they can be dirt cheap if buying used. For example, I bought a used Inwin PE689 for, like, 30 euros on the local market.