Scenario for interconnecting 2 Unifi's

Noob here..

I have two UniFi's - one operating as a fancy AP , the other still in the box.  What I would like to accomplish is to have 3 WiFi networks running seemlessly operating across both UniFi's 

The WiFi networks I need are:

1. for trusted devices (access to network resources, printers, NAS & internet etc)
2. for IoT devices (access only to internet - but some devices will also need to talk to mobile clients on network #1 - think LIFX, Sonos)
3. SSID for Guests (access only to internet)

From reading a little about the edge router, I think I could setup this scenario without issue, but would like confirmation from the community before I rush out to buy.  The router would be connected to a basic DSL modem.

I've taken a look at this thread: https://community.ubnt.com/t5/EdgeMAX/Multicast-Sonos-Phorus-amp-Play-Fi-Broadcast-255-255-255-255-lt/m-p/1268559#M67208 and while some of its over my head, it looks really encouraging.

So...

- can I accomplish it

- and what would be the best approach to setup (happy to learn, just need to be pointed in the right direction - eg: Multiple DHCP or Multiple VLans or both ?)

Thanks

Garry