Need Help, Design a Solution - our Web Application should be able to gain access to MS Sharepoint Online (File Up/Down Load)

Hi,

i Need some help to get in the right direction or maybe to the right Forum.
We have developed a web application that is hosted by third Party.
The application runs on Windows IIS.

Customers are able to logon to our web application and use it as designed.

Now the Marketing want´s a new Feature: some way to upload / download documents, files, pictures.
The data should be secure between different customers.
Each customer should have his own "file space", no customer should be able to see other customer data.

The data should be "put through" to Microsoft SharePoint 2013 Online Services.
Data that the user Uploads should be send to MS SharePoint and i think we must design our own Access control for SP Access.

Now for my design Problem/questions

1. Would this work?
2. What SharePoint Online License would be usefull (what to buy, single User Access, for every customer a user)?
3. What Technology would be the best for the IIS Application?

I am no Web or Software Developer, but i have got this Task today ...

Any help or Information would be great.

Thanks

Jürgen