Backup

Hello Everyone

I currently have six server :

2 SQL 2012  servers with always on availability groups

2 domain controllers with DHCP installed  (To provide domain and DHCP redundancy)

1 File server

1 server that has a couple of work related applications.

I am looking for a good planning and solution for backup

Is one of the option getting a NAS and using server windows  server  backup to backup entire server images , backup files and SQL databases?

If windows backup is not very efficient to handle all these servers , what other good third party backup can I use?? And please give me hardware recommendation for getting a NAS (size , price , number of disk…)

Thank You.