Time machine: network hard drive vs Time Capsule
I had my Buffallo NAS configurated as Time Machine Backup NAS. From one to the other TimeMachine told me that the Backup Volume was corrupt and could not be recovered.
Possible Reason for the corruption is the filesystem, which had not the full capability of ntfs or mac os extended.

Right now I try to use a Synology NAS which has a build in TimeMachine support.

But if you want to be on the save side with your backup I would advice you to buy a TimeCapsule which has the best compatability with the TimeMachine.