Do yourself a favour and spend a little on the smoke detectors. Their job is to wake you up and get you out if there is a fire.
The Cavius photoelectric smoke detectors that Mitre10 sells are really good. They are fully tested before they leave the factory, and very seldom have faults.
$49 for the 5 year battery one. They also come as wireless interconnected alarms with 5-year battery.