How to organize joint work with documents via the Internet?

Question

Document flow

Colleagues, help me find a solution on how to do this, maybe someone has experience.
There is a legal Internet company, employees work in different offices or from home.
Employees are not IT people and do not follow IT instructions (like storing files in Truecrypt, updating antivirus, etc...). There is no time to control everyone, of course.
Now: a set of software is used on the computer of each employee (mail client, office, Bitrix24 desktop, Jabber, Skype, etc.).
Tasks:
1. Need to collaborate on documents (little trust in third-party services)
2. High degree of confidentiality (if possible, do not store locally at all).
3. Flexible rights management.
Tried:
1. owncloud - problems with synchronization, especially when all sorts of special characters in names or long paths.
2. Bitrix24-disk - no words... (files disappear where they don't need, appear where they don't, etc.)
We look at the following options:
1. Banal Google drive (but this is data storage on a third-party server, not on your own)
2. Citrix Sharefile (we know little about it)
3. S3 - they say it's expensive?
4. Rent a windows server and set up a terminal service, let users work through it.
Most of all we tend to the 4th option. Maybe there are other solutions?
Thank you!

Answer 1

2. High degree of confidentiality (if possible, do not store locally at all ).

that is, you want to edit only in the cloud? Then they can still drag it to themselves.
We use Google drive, it's fine. There are troubles with giving away access to new/leaving employees - they become the owners of the files that they uploaded themselves and can steal for themselves. Therefore, everything is implemented not through personal accounts, but we create our own for everyone.
In principle, if you do not have many users, then you can get hold of Google Apps and maybe it will be quite good. (I haven't tried).
p.4 also works fine, with its pluses and minuses