To be honest, it would be false economy to just buy the HDD that matches your current need, Totem is always increasing the detail of their cards, so the file sizes will get bigger.
I currently use a 4 TB external HDD for the collection, but as one drive (from two separate PC's) seams to have failed, I will be replacing that one with a 6 TB HDD, but your decision to go with an 8 TB is a sound one.