What you should do depends on your experience. If, as I suspect is the case, you have little or no development experience, the best suggestion would be to get a reputable web development company to build it for you. However, do you have any experience of writing a specification for a development? If not, you may also need to employ an experienced project manager, to formalize the user requirements which form the basis of the legal contact, and to oversee the development. Do not underestimate the importance of a formal written spec. Without it there is little likelihood of success.
If that is ruled out on cost or other grounds, you may wish to consider a self-build using a CMS, but I would strongly suggest caution. What you are asking for is not trivial. You could invest a lot of time and effort and end up with something not fit for purpose. And then if you decide to employ someone to sort it out, you might find that it costs more than binning it and starting from scratch!