Having a site that clients can login into as well as accountants would require Security Assist. The pages that the client and accountant view and interact with would be created by DataAssist. Finally the upload functionality would be handled with the newer version of Universal Email.
There is no guide that we have that is specific to this setup so it will be a custom implementation but from what you have described I believe these are the tools that you would make the most use of in this project. You could even add in functionality to send an email to the client once their accounts have been reviewed.