Create a chat web application

We'd like to create a chat application based on a well-validated, open-source chat solution such as rocket.chat. Features (by phases - p1 = phase1, p2 = phase2...): - (P1) Users will engage in real-time and offline chat with other users. We already have a real-time chat solution that could be used if needed (websockets..). - (P1) two-users in chat room (later we'll add group chats) - (P1) Signin/registration NOT done via this chat solution but by our signin process (need to authenticate though, we can provide API to our REST server). - (P1) Enable guest-chat - (P1) Multiple end-points support (for users opening application in multiple tabs, multiple devices...) - (P2) Searchable chat (search within chat-room, within entire chat history) - (P2) File sharing using an external storage solution (AWS...) - (P2) Client side emoje's - (P3) Group chats Other important technical factors: 1. Open source chat should be FREE and easy to manipulate. We do not want any licensing trouble :) 2. Preferred open source would be based on javascript / java in the backend. 3. All UI / UX should be customizable. 4. Solution should support up to 1K users actively chatting simultaneously. 5. Test suite is a MUST Important proposal factors: 1. Only English speaking partners would be considered. Access to English speaking developers is a MUST. 2. Please provide a brief explanation of your proposed solution. We will talk about the details in an in-depth conversation. 3. Previous experience in similar projects is an added value 4. In the proposal, please include your pricing for P1, P2, P3 separately. Thanks and good luck :)