What are the best ways to edit documents collectively?

Question

Project management

There is a team of lawyers (about 50 people) who provide legal assistance on documentary issues related to organizational activities - in other words, they help a new organization develop the necessary package of documents that comply with current legislation. One of the necessary tools is the collective work with documents.
From the wishes - the security and availability of documents, equated by the team to the location of the server inside the country. For these reasons, Google.Docs and Office365 clouds were swept aside. Ideally - your cloud of documents on your server, but sharepoint - at the start is too expensive.
What means of simultaneous work with documents will be convenient to use? Which of the following would you recommend, or vice versa, would you recommend?
Under consideration: SharePoint server; Bitrix24; VLSI. Maybe there are other options?