Use rSync instead of iSCSI, FTP or SAMBA …
There are rSync client and server for Windows out there. That is fast reliable and it's designed to work over unreliable network. Plus the algorithm of rSync will only send what have changed in your file instead of sending everything back. So your file of 1.5 GB could be backed up in less than 100 KB if only small portion of the file where changed.
I have several rSync configurer on Windows as server and client and it's working great. Then you schedule a batch file.
Instead of using IPSec VPN you could use SSH tunnelling again available a client and server for Windows
Here is the site you should look at: http://itefix.no/cwrsync/
If your more serious about backup, you should also check Ahsay Online Backup Server. That's what we use at work, we have an offsite server in a datacenter running Ahsay OBS there's about 150 GB of data there backup takes less than 30 minutes each day... Ahsay technology is based on rsync. We have used Storegrid for a year and the backup set got corrupted ... We have switched to Ahsay for that reason.
MageMinds