Linux – Using BackupPC to backup Linux server to a windows network drive

I'd consider having a look at the Backup Exec System Recovery Server Edition (Windows) and Backup Exec System Recovery Linux Edition products. The Symantec Backup Exec System Recovery Virtual Edition product might fit your ESXi host better, as well, so have a look at it. I believe they hit all your requirement points. (I'm not making a personal recommendation, but these look like they might fit your bill. I've worked around the windows product and, frankly, don't like it. That's probably my personality and preferences talking more than anything, though.)

You don't necessarily need to have the computers establish a VPN connection on a timed basis. BackupPC will to backups on laptops and such whenever they are reachable. Partially, completed backups will be finished next time it connects.

Initial backups require transferring all the files being backed up. These will be the longest backups. Once that is done all subsequent backups will only require the differences from the last backup to be transferred. About 1% of the files will be transferred to validate the contents on the server.

You can use the alias to provide an alternate address to contact the fixed PCs at remote locations. You can also override the commands used to determine if a PC is reachable on a client by client basis. You can also run commands before and after a backup. This should give you lots of flexibility to handle remote PCs with fixed.

Ideally, you want the laptops to be reachable from time to time. However, if you can script a post to the web interface, the laptops should be able to trigger their own backups whenever they connect.

It may be better to install VPN software on the laptops and let BackupPC pickup the laptops when they are available. BackupPC will email the user specified in the host's user column if the backup gets too old.