you 'could' do it this way but id suggest its a problematic way.
A better way would be to use a 240v relay and connect this to the incoming supply. When that opened it could open your gates straight away. If you would like a delay then id add a timer to hold off the relay.
an external warning?
Well the simplest would be a mains relay from the light and an external sounder and power supply.
But id recommend the gjd system, or an full control panel would be better.
Depends on their budget and needs really.
its not 180 deg, i dont know of a single head pro unit that is. Its usually only the low qualioty stuff that is.
but it is 12v, with adjustable times etc
Have a look at the product page download the data sheet
12v dc?
id so the only unit that will do 180 degs is the voltek twin head. But this doesnt have adjustable times etc. Apart from the opal elite (gjd) none have a programmable time this is usually done by the controller
Id do the mains first as its easier to connect that quickly, ie push fuse in. A clean power up is essential. Howver as soon as possible the nattery shoudl be connected. SOme panels need the battery to operate the protective device (ie the lower voltage fuse) if you have a wiring fault you can end up popping the panel with out the battery connected.
To be 100% safe fully charge the battery from the panel with nothing connect to the pnale (other than the mains and the battery)