I have two NASs on my home network: an old WD MyBookLive which the *DWO's laptop backs up to and a Buffalo NAS with RAID 1 mirrored for mine (RAID type is software configurable). I had the two original drives fail on the latter about a year apart causing major grief as the RAID hadn't recreated properly on the new drive - lesson learnt - I have now have the software perform a full integrity check fortnightly.
Advice I have been given is have a two tiered system one doing active backups and one long term storage copying the former to the latter at intervals.
I would suggest you need much more than 2x the required storage. Do a full backup at intervals and incrementals for new/modified files in between (I have actually configured mine to back up separate tranches of data in sequence because of runtime constraints), keeping a set number of copies of the full backup and deleting the oldest and incrementals which have been included in a full back up at least twice. If they are like mine the accompanying software will enable you to configure this fairly easily. I have run into issues with corrupted or incomplete backups in the past. I did experience a complete drive failure on my laptop when on holiday and with some fiddling was able to restore everything up to the point I left. I'm considering moving to one that uses SSD if I can get sufficient capacity at a reasonable cost.
*Domestic Warfare Officer