My experience with iCloud is that there is no way to predict what will be kept locally and what will not. It will just manage to leave space available. But even when the SSD is far larger than the space taken by the whole iCloud, it will most likely not keep anything locally.
So basically, I cannot rely on iCloud to have a document ready to browse offline in my next train ride, flight, or what not. So, I would not take decision on the amount of storage needed based solely on the space required to fit the whole iCloud.
Now, in general, if I were buying a Mini now, I would have a hard time getting 64 Gb...