Setting up a new enterprise network on a shared gigabit fiber.

Hello! I'm responsible for setting up a network on a new location for our small business. I have more of a coding background rather than networking, but being the only tech-literate person around I get throw everything else too. There is a gigabit fiber going into the property, and there is already another business set up on this connection. What I need to do is set up a new LAN completely separate from the existing one. Before I start buying hardware it would be nice if someone can spot any issues in the setup that could cause me trouble down the line.

There is a fiber converter connected to the other companies WAN-router. I figured the first step is to connect a new router right after the fiber converter and just run this in bridge mode and no DHCP. Both businesses connect their routers to this one with static IP-addresses set up.

From this router I can then run our DHCP, NAT, firewall and NAS. I'm planning on running 5 UniFi APs for full coverage and seamless handover, hopefully with PoE if I can get the budget for the right hardware.

Couple of questions: I need a total of 16 wired Ethernet-connections. Are there any up/downsides to getting a router with 16-18 ports and skipping the switch altogether vs getting a smaller router and run it all via a switch? Does it matter in terms of Wifi performance and client handover if the APs and router/switches are from the same company or not? Is it better to split the connection right after the fiber converter and skip the first router altogether? I assume that would entail messing with the ISP to give us several WAN IPs etc.

Thanks for any help! I hope this isn't too low level for this sub, but if it is I apologize. =)