There is a good thread here on using Synology as a Linux target:veeam-backup-replication-f2/backups-to-synology-nas-t1961.html
I suggest you read through the entire thread as the initial posts become outdated with new Synology updates. I have a DS1815+ which I am currently copying to using iSCSI. Even though Gostev says NAS drives are not recommended I do believe that NAS drives with iSCSI don't fall in to that statement.
On your off-site target you will need to be able to install the agent (Linux or Windows, if using Synology obviously Linux) and have a tunnel to that target. If you are thinking of using the Synology I would highly recommend you try out the steps mentioned in the thread above to get the Linux agent installed on it before buying it for your off-site target. However, running a Linux or Windows server with a nice RAID array might be a better option especially since your costs don't increase all that much and you have hardware that's up-gradable. That's the way I'm leaning. Having a ESXi server offsite which will be replicated to using the WAN accelerator (through a VPN tunnel) and then backup copy jobs running to a VM server under that ESXi host. This way we have our production data, our backup data locally (hosted on the Synology), a production server ready to spin up off-site, and an off-site backup repository that backup copies are sent to (this can be Synology but since we already have a server off-site and our data isn't humongous I am leaning toward setting up a Server 2012 that will host the VPN, the WAN accelerator, and the RAID array for the off-site backups).
Gostev, please correct me if I am incorrect on the statement that iSCSI NAS is perfectly fine (that's my current setup which seems to work well).
Edit: I should add that the CIFS (shared folder) method of moving data to the NAS drive is absolutely horrid. So you don't want to use that.