Using apple and Ae's for multiple room audio

So this is the set up that I was thinking about and wondered if anyone had any input, pointers or no-no's !
(oh by the way this is to try to keep to a low budget - can't afford Sonos in each room)

Situation is this:
New build house, not even wired yet.*
I want audio in 16 of the rooms (over 8 zones. I'm happy to have some of the rooms playing same audio)

At this point I am getting each room installed with in ceiling speakers, prob just 40/50w ceiling speakers, left and right for each room.*

Each room will also have ethernet (cat6).*

The Audio Source (I want only 1 source) as an iMac.*

I was thinking this set up:
iMac in Family Room
Cabled to the plant room from the digital audio out on the iMac
The iMac is connected to the home wired network
iMac and router connected to a 24port switch
8 x airport express's connect to 2 x 4 zone amplifiers
Each of the 16 rooms speakers are cabled back to type plant room
Each corresponding speaker / room connected to the zones on the amp
each AE will be on the wired network from the 24 port switch
This way I was hoping i could control all 8 zones over AirPlay from iPhone iPad etc.....

Do you think this will work?

Some questions
any speaker cable spec suggestions? AWG 18?
Do I terminate each room speakers in each room or chase all *back to the plant room?

Or is there a much simpler option for all this at I am missing!*

Thanks.....

Only 5 AE/AEBS can be controlled by Airport Utility and you need to look at this for 16 room capability. ELAN HOME SYSTEMS: Entertainment Sources
Also hear is the best guide on speaker wire. Speaker Wire