New office network, firewall, nas

[tecno2018]

hello to all,
i'dd like to solve my office network.

actually, i have two LAN:
1 without internet acces: there are 1 pc(with important data), 1 printer and 10 tvcc IP camera with NVR
2 with internet access: 4 computers use to surfing the web for researching, sending email and writing documents, 1 printer and some mobile devices connected by wifi

i draw a simple schematic for my idea.
i think to fit a NAS for backups (COMPUTER A and COMPUTER 1,2,3, NOTEBOOK 1) but i do not want to fit my nas into a LAN with internet(because virus or ramsoware/cryptolocker) but i want to share some NAS data with other PC's.
There is a very secure solution for this?
Sorry for my bad english