Bittorrent Sync is not a cloud storage solution. It is a way to synchronize files between devices. I am not aware of an iOS version of BS so that leaves only the laptop and desktop to sync. For the sync to work both devices need to be turned on and have an internet connection or share the same LAN. Do you leave your desktop on when you go to work? I don't know how Quickbooks and Sage store their data, but if it is in a binary blob a lot of bits need to be transferred with every change you make to your data. As for your external drive. You would need an extra backup procedure for that since BS only manages 1 copy of a synced folder on a device.