Each area with speakers (probably two per room but could be more) would be a zone that would need to be powered separately so you can adjust the volume individually and have the option to play different music in each zone..
How much power you need for each zone depends on the speakers, the size of the room and how loud you want them to play. Most in ceiling speakers are designed to be used with 40 to 75 watts per channel.
Older house music systems have become obsolete so using the old "control unit" may not be the way to go..
You can use a Sonos ConnectAmp for each zone and control it with the free app. Much more flexible and easy to use.
Denon Heos Amps or Superlink with any amplifier would work well too and has some advantages over the Sonos.
A custom installer in your area will be able to check the speakers, control unit, and wiring you have and set you up with the correct equipment to suit your needs.