I require a controller that will take as inputs of time and temperature (entered by operator) for each of the 5 plates. Temperature will be maintained for the duration of the time inputted, when time has elapsed temperature is switched off and an alarm siren is sounded and the light tree goes from green to red.
Are you actually planning to have the Arduino control the temperature? You talked about using an external state subsequently, in which case the Arduino doesn't need to do anything except turn the heater off and switch the lights over when the time has elapsed. Depending on the length of time and timing precision you may not need to implement your own solution for that - all you need is a timer that will turn something off after a delay - use that to switch a twin pole twin throw mains voltage relay that powers the green lights + heater in one position and powers the red lights in the other position. You can buy domestic timers to do that off the shelf for a few quid. Using off-the-shelf components to control the temperature and timing would give you robust and user-friendly controls and save you a hell of a lot of work.