The internal Drive is not user-upgradeable. You need to buy it with the size Boot Drive you want, because compatible replacements may not be available. There are no additional internal drive slots. 1TB is far too tiny to hold all your movie data -- You WILL need a large external ThunderBolt drive or array. You can use one USB-3 drive full speed in both directions before the USB-3 Bus is saturated.
I strongly doubt you need so much RAM for Photo and Video editing. Users who are installing this much RAM are typically doing large simulations such as weather forecasting.
If Apple only supports 64GB, you need to find a vendor who will not only sell you the RAM you need, but also support you through the installation process. Otherwise you may end up owning some very nice modules that work perfectly, but do not happen to work in your Mac.
16GB and larger modules are Registered (they contain a Buffer Register) and cannot be mixed with regular unregistered DIMMs. The only reputable Vendors I can suggest are OWC/Macsales and DataMemorySystems. Crucial, as usual, do not offer anything outside of Apple's recommendations because they have no lab full of Macs to test with.