Customize Existing Forum Software

I would like someone to customize an existing forum software for use on a business website. The forum would be used to interact between lawyers and their clients, so security is of the UTMOST importance. Everything would need to be protected from web indexing and password protected so that users have access only to certain sub-forums or threads. The first post of each thread would be started by a client filling out a web form. Then the details of that form would create a new thread. Users would also need to be able to attach files to the posts. The ability to archive is also essential. I am aware that there are a number of open source forum solutions, but am willing to pay additionally for software if that is necessary. Please contact me for more information, I believe it shouldn't be anything groundbreaking if you already have experience with web-forums. ## Deliverables Choose the forum software you are comfortable with, and customize it for me to be secure and to make one form. No-one should be able to view the forum without a password, and each user needs to be approved by an administrator. Alternatively, the administrator can make the user accounts and hand them out. It is a small forum for use of less than 50 people. Ideally each user could access only certain sub-forums or threads, depending on what the administrator specifies.