You'll need two Synology units, the main one could be the 514+ which costs 540 euros. If you start with two Western Digital RED disks, 6 TB each, that's 2 x 260 euros. You'll then have 6 TB if you mirror them.
For the backup, you could get the same setup. But you can also get a cheaper/older Synology, for example the Synology 416j which is 300. Getting that with one hard drive is fine, with enough bays to extend it later.
That would bring initial cost to 1620 euros is 1770 USD.
Backing up a symbology to another one is documented here:
https://help.synology.com/dsm/?section=DSM&version=5.2&link=BackupApp/repository.html
Furthermore, there's a standard software module which lets you back up the Synology to their cloud. There's a monthly cost involved but I don't know how much that is.
(Note that the Synology series has a particular way of numbering. First is the number of drive slots, then the year it came out, then there's an indicator for the speed/feature set. Thus 514+ means: 5 bays, came out in 2014, and the plus means business. There's also j for junior (cheap but slower) or "play" for media purposes like hooking up to a TV.)
This is just an example setup and I'm not an expert. I hope other commenters can jump in and correct/optimize the above.
