Hmm maybe I should buy One Drive storage only since I have MS Office downloaded on my computer already.
I think you're overthinking it really. Just pick a service and go with it. They all have ups and downs, but the differences aren't big enough to warrant this amount of consideration I'd say. I personally prefer iCloud Drive, but they all offer cloud storage and you'll achieve your goal with either. Going with the cheapest is probably also a good solution for you
If you pay for the Office subscription, you already have OneDrive storage.
His Office is not subscription Office. It's the retail copy - stated in a previous post. AFAIK that doesn't come with OneDrive.