Wow, 8GB is a pretty powerful server, I think the 4GB variant would also be enough. So there are passively cooled cases made of aluminium, that might be a good thing. But I wouldn't install a fan, they break down at some point. An aluminium case with aluminium bars on the inside, which then reach down to the components that need to be cooled, that would certainly be enough. I would take one of these: NEO Case for Raspberry Pi 4