Getting to ICN from Seoul City Centre is going to take AT LEAST 90 minutes either via the Airbus, or combination of metro/suburban train. Personally I'd suggest going for ICN at noon. You will have plenty of time to get to the airport and also be able to get up at a reasonable time as opposed to the early hours. Also if you are comfortable consider using the metro/suburban, much cheaper and much more comfortable in my view. Another option is to consider staying in one of the many hotels surrounding ICN and simply pop into Seoul the evening before for dinner/sightseeing etc.