I am an experienced FileMaker developer and have been developing applications using FileMaker for the last 12 years ranging from complex CRM systems to Website Content Management systems as well as solutions utilising FileMaker Go.
I have developed several CRM/ERP systems which included the development of Invoicing, Quotes,Sales Orders,Stock Control, Suppliers Invoicing and Purchasing and Work Orders modules. In some insatance the Work Order (Jobs) module was also used as part of a graphical Scheduling tool using the native FileMaker Webviewer.
One of my most recent projects was to develop a Web Portal using FileMaker Direct for a Fire and Security business who like you needed to be able to allow clients to upload and download documents relating to the service jobs they have requested among other functionality.
Security is achieved using a Web User table and a seperate database file which only contained layouts specific to their requirement, this allowed them to restrict access to the Web Portal to only the clients they specified and only to the records that relate to their company.
I think your budget is a a bit low for a complete solution based on your project description and attachement, therefore I have bid to create the document functionality required and implementing the security features.
If you would like to see some examples of my work then I would be more than happy to arrange an online meeting with you to demonstrate them.
Regards
Martin