Hello,
How big are the files?
Email is not secure by default. I oculd encrypt the emails using PGP, but then again, if the files are large, email is a bad solution. I suggest uploading the files to a secure location (maybe even encrypt the files) and send you the email with the secure link. Payment gateway integration is easy. If you want this to be secure you will also need a SSL certificate and a dedicated IP. IF you really want security, you can't go wrong with a dedicated server (NOT A VPS), as if you are on a shared host, one hacked wordpress can expose your precious files to the hacker. Let's discuss your project in detail. Thank you.