I want to back up my VMs over the internet to a machine that's at least serving SSH, could serve NFS or iSCSI if necessary. What's my best strategy here?
Add a Storage (NFS or iSCSI) which points to your desired backup location (Datacenter->Storage->Add). Now, when you're making a backup job select the added remote storage as destination.
You could also make a backup job (Datacenter->Backup), so you automate things and doesn't have to worry about it.