Hi!
If the NAS are Synology (Most I've used works like a charm) you can use the Backup option that uses rsync inside... But you can compile Bacula into some Synology Devices too... (Google is your friend!)
I have a small setup like this working:
HP Proliant MicroServer (AMD 1,3Ghz/4GB/2x250SoftwareRAID) (Bacula-DIR, Bacula-SD) + Ethernet Gigabit PciE4x Low Profile (Cheapest config!)
Synology NAS DS212J with iSCSI exported to the HP Proliant server (/backup)
All the computers in the network has a Bacula FD installed and saves their backups jobs to the HP Proliant MicroServer thats saves this data to the Synology via iSCSI through the second ethernet interface.
The HP MicroServer it's not a powerfull server but it's so quiet and cheapest than any other setup, and you can expand it to 4 drives and up to 8GB RAM.
Maybe the best option could be do the job with the tools that offers your NAS ... I don't know ;)
Best Regards!
Javier.