OK, good goal.
I'll sidestep the issue of how many speakers, which tells how powerful an amp you will need, for now.
And it's been over 15 years since I set up any Informers, but this is what I remember from then: Then ones we installed had a set of Common, Normally Open and Normally Closed relay contacts, maybe even 2 sets. And of course a pair for connecting the audio out. We found that there was a fair amount of electronic noise from the device driving the audio pins, so we had to route the audio thru the Common and Normally Open of a relay to keep it quiet when not alerted. Maybe they've made it better since then, YMMV.
Those are 1 watt output into an 8 ohm speaker; manual says 600 ohm output. That would go into a Line In port of your PA; putting it into a microphone input would overdrive it, unless you put a bunch of attenuation in there.
Mattsenft has good points. You need software & cable and knowledge of frequency and tones, and a few other important items.