This is exactly how I've got mine set up. I'm writing a "how to" at the moment for the Blue Ring and will post it when I'm done (need more hours in the day ATM).
Basically, you need to do this
Set up your users and assign them to a group. Ensure www-data is a member of the group, or better still make your uses a member of the www-data group. Set your home directory for each user to the share you want to use.
Set up sftp default mask and folder write permissions to 775
Check to make sure samba permissions match this
Open a port on your router/modem to connect from the internet to port 22.
Set up DynDNS or similar so that you can log in from the outside world via a URL.
Use a sFTP client. I use WinSCP and RedDrive. RedDrive is good for those not technically inclined and allows you to essentially map your share as a windows drive, and doesn't allow the user to navigate up higher than their home directory.