A website with an online portal for clients to upload private documentation on to

I am looking for a custom-built website with an online portal for clients to securely upload private documentation on to. Security is essential, so I am particularly interested in strong encryption features and multi-factor authentication options for the online portal. Additionally, I would like the website to have an automated backup system for added safety. I do not have a preference when it comes to platforms for the website so I am open to a custom-built platform for this project.
The website requires a unique login for each customer that can then be shared with solicitors and other parties involved with the transaction. The website is related to property purchases.