At the moment Riter is not available for self-hosting - while it's in beta, we'd like to first check and debug its work properly on our side. When we release (in about 4-5 months), Riter's Enterprise plan (https://riter.co/#pricing) will probably be a good choice for your requirements.
We provide a dedicated space for each company's data, so that you remain protected from third-party interference. The distribution of user access rights and roles prevents accidental or deliberate data corruption by your employees. Enterprise plan will include two-factor authentication, SAML 2.0 support and custom OAuth 2.0 integration. Our team is constantly tracking vulnerabilities and fixing them. We use secure data transfer protocols and reliable servers. If you are interested in something specific - we are ready to answer any additional questions.