It all comes down to what do you want to prioritize here. If you want minimal downtimes in case that there's a power source failure of any kind, then you probably want ext4 which will give you the fastest fsck times. Or, you might want to check into sqashfs on a flash drive for your rootfs and use whatever else for writable parts (/tmp,/var/log/, etc.), and update only when strictly necessary (GLSAs can probably help you there). After all, as someone else said above, this machine just needs to do one thing, and do it well. If you plan to make stage4/5/6 or whatever the trend is nowadays to name it, you don't even need portage or a toolchain in that box, and having it will only be a security risk since some rootkits comes in the form of a kernel module that needs to be compiled for your specific kernel and architecture (eliminating the kernel sources and the compiler you sort that out from the very root).
In any case, the cpu won't be a limiting factor or a bottleneck, whatever your definitive choice shall be. -- Jesús Guerrero Botella
