Backup / Recovery Best Practices

The problem you identified is a lack of local disk space. To work with what you have, you could plug the NAS into a local USB 2.0 port and run the NTBACKUP directly to the NAS, or try it through the NAS share. You would no longer need 800 GB of local disk space to hold the backup, you could restore straight from the NAS volume. The end result is a minimal number of backups held on disk. Look into deduplication. I also don't see a 2 hr RTO getting met with this method, though. You need a more robust recovery plan, as D-Gee and others commented.