Hi,
I'm a full-stack web developer having 7 years of experience in a number of Lamp stack.
I'm very familiar with chatting functionality development from scratch.
One of my previous project was very similiar to this task and it was the customizing of the open source chat platform "Zulip".
That client required to customize Zulip chatting front-end same as Slack and I've finished it perfectly by using React.js.
If you have already finished chatting method, I can customize its front-end by using your preferred front-end frameworks and languages like React.js, Angular.js and Vue.JS.
I recommend to use React.js, the most trending front-end library.
Also, if you don't add much code, I can use only CSS and UI frameworks to add least code on your project.
This attached screenshot was one of my previous chatting project.
With my experience and background, I can surely handle this project.
Hope to hear from you soon.
Patric