Texecom, Pyronix and Honeywell seem to be preferred but maybe the Texecom & Pyronix are easier to program? As for the zones, you would usually split them into areas or detectors. e.g
back door contact would be 1 zone,
living room PIR another zone,
garage door contact another zone,
conservatory,
window sensors,
smoke alarms,
floor sensor etc...
So work out how many areas that you would like to protect/monitor and then go from there. Also you need to take into consideration if you will be able to run cables to those areas or will need wireless (or a mixture). Also keep in mind that you might have rooms that don't have any external access via door or window so these might not need there own detectors. Also think about features you might want, e.g. call/sms alerts, home automation integration, an app to remote set/unset, keyfob entry, multiple users with different access levels and so on...