Need help for small office network

Hello everyone.

I've been tasked to build a network for a small office. No one here is experienced with computers, let alone networking. I told them I'm able to build computers and suddenly they tasked me to fix the chaotic network we have here because they're too cheap to get a pro to do it.

So our office has multiple CAT6 LAN ports where we can access our internet. For years people at our office has installed 4 wifi modem+routers that are independent to each other. As a result, we don't have a unified local network and even had to resort to connecting our printer to a global network. The NAS we have is only accessible by connecting to the modem+router it's connected to.

Our office is quite large, not sure about the exact size. Everyone here uses wireless connection and a maximum of about 150 devices connected to multiple wireless routers. Question is, how do I go build an appropriate network for something like this?

My first (naive) idea is to just buy a consumer-grade modem+wifi router like the Nighthawk X8, connect it to the wall LAN port, connect the NAS, and other wifi routers to it. We will probably install an unRAID machine later on for cloud storage accessible locally and from outside the local network as well. My boss is worried about security, as our campus recently got hacked from an unknown outside source.

Would something like this be possible and at least adequate? Sorry if the answer to this question is obvious to you pros, thanks!