Wordpress Website with Customer Login

Seeing as this is an accounting firm, their first and most important priority is security. They MUST keep all information confidential and secure. Often times a client will phone in and ask for a copy of old paper work (i.e. a copy of an old T4 for an employee). Some information, while easy for them to locate and send to us, is time consuming for them to have to do. They would like a way in which clients can safely and securily log into their own 'filing cabinet' and pull out what they need. The accountant would determine what type of info can be stored in the filing cabinet and what info cannot. ** Primary concern is having a system in place that double checks that the file being placed into the filing cabinet is, in fact, for that person. ** Most often a person's SIN number or Business name and number is what items will be filed by; however, we need to mitigate any risk that an incorrect file could be placed into a client filing cabinet. The website itself need not be fancy. They need the basics of contact us, where we are, who we are, our services and then the secure log in area that takes the client to the secure 'filing cabinets' of information.