It just depends on how long you plan to keep the machine and how much work you do on the machine.
If you want to keep the machine for a very long time and want to use it as your primary machine, then 64 GB is definitely not enough. But if it's a secondary machine or a light work machine (i.e., you only use Word), then it's fine. Remember that Modern apps can only be installed on drive C. A memory card will not help. Install things like games (some Modern games can take 2 GB each, let alone traditional Windows games which can take multiple GBs even though traditional Windows software can be installed on other drives besides C

, Adobe software, etc., and that space will disappear quickly.
If you're using it as a lightweight computer or as a tablet where you're okay with constantly installing and uninstalling software over time, then you could live with a 64 GB machine. People with tablets kind of treat software as disposable. People with computers tend to focus on certain software and keep it forever.
Get what you can afford now if you plan on keeping the machine a long time. It's not like you can swap out the SSD storage on the SP3. What you get is what you'll have forever. The microSD on the SP3 maxes out at 128 GB (though technically Microsoft certifies it for 64 GB because of the unreliable nature of microSD cards at higher capacities), and again, you can't install Modern apps on the memory card.
OneDrive helps things out a bit but you need a great connection to make it feel seamless or smooth. I back up a lot to OneDrive, including videos. People complain to me the videos I share on OneDrive buffer a lot or take time to get sharp (you know, it's kind of blurry until it's buffered enough to be sharp) compared to the other services I use (though I stick mostly to OneDrive and Youtube for video storage due to the vast capacities I have on those).