Hi,
I really think you're trying to re-invent the wheel. This could cost you a lot without any certain results.
Also, I'm not sure what others will propose, but do you think you can recreate Dropbox for < $1000?
Anyway, I have a real solution for you that can do the job and is very reliable. There is an app called OwnCloud; this app is open source and is similar to Dropbox.
I'm using it on my own server as a storage for my family, and also configured it for many of my customers. It also has an android and IOS app, in addition to windows app that goes with it, so you can sync anywhere.
You can fully customize user accounts (available storage size, number of files, permissions...).
I really advise you to take a look at that application, and if you like it and don't feel like trying to do it yourself, please let me know and i can take care of the installation, customization, and even train you so that you can administer yourself without any additional help.
Please feel free to reach out for me if you need additional information.