I wonder what the difference is between "make available online-only" and "make available off-line" is?
I would have expected a "make available off-line" would copy a file to the device you are using so you have that file available off-line?
Have you looked at/considered Microsoft One Drive? I used to use that a fair bit and found it quite nice for making sure a file I worked on on one computer I could access on my other PC. You can chose if you want the file synced or not, off-line (so saved to HD as well as cloud or just on-line access.
Price I don't think is bad at all - you can get 1TB of storage for £60 and that also includes the main products of MS Office as well. Goto the "family version" for £80/year and you get 1TB per person - so if Liz needs storage as well, gives you 2TB in total (and can have upto 6 people in the plan).
This is actually a lot better value than the extended Google Drive thinking about it at £60 a year for 10 times the storage of Google.