"should I build/purchase a NAS? Or are my current backups enough? " Seems like enough to me... grin. Maybe consider offsite backup? A fire would hurt this backup plan.
(1) for me there is value in the backup *not* being direct connected. If anything gets thru my firewalls, it can't destroy the backup too. I backup to external drives then pull the plug. I don't automate backups though, which is my bad.
(2) if you have a spare PC, and want to build a NAS then FREENAS might be worth researching. http://en.wikipedia.org/wiki/FreeNAS
(3) A dedicated NAS box might save you enough $$ on electricity and hassle vs. the old PC that it's worth it. And might perform a bit better.