Advice for Membership site/form upload

We have a website that we would like to have a login area for clients/potential clients.
We want them to be able to do the following:
-login (we may provide them a registration or let them create their own)
-Click a button to fill out a form with information and upload files that may be needed by us (we are thinking of using Gravity Forms) This will be basic client information and some excel sheets or invoices,etc.
-Access documents or information that may be specific to them. We may place enrollment paperwork or plan information specific to them to download. We do not want clients seeing other clients information.
-Be as secure as possible, which I am expecting more to be on our host level

If anyone has any advise on the best plugins or approach it would be greatly appreciated.