Hi mate
As regard the mechanical aspects of the aircon you need compressor,condenser,pipe work dryer,expansion valve,evaporator,pressure sensor and thermostat/cycling switch,control panel/on off button.air con equipped ecu,s had a function to disengage the compressor clutch output under load so may be different(under wot to give max power).the dryer MUST be replaced when the system is open for more than 10 hours.you must ensure all parts are dirt free and free of moisture.dont forget that the pag oil in the compressor is hygroscopic (pag oil) this absorbs moisture once open to the atmosphere which will in turn contaminate the system and cause damage.correct vacuum levels must be reached when commissioning the system and keep in mind that oil travels throughout the system so the correct amount must be calculated for the system as a whole.i must say there are too many people who deal with this stuff who haven't a clue and don't know the real ins and out of air con as a whole.it needs to be done properly and if it's not be prepared for it to fail,especially in Australia where the high side pressure will peak.
DO NOT gas this with a can from halfords or any other factors for that matter,they are nothing but shite.
Best of luck though
Eoin