Going to Busan directly after arriving to Seoul- need help with transportation planning

Going to Korea for the first time for 2 weeks in late May and my first stop is Busan. My flight to Incheon is scheduled to land at 3:45pm. I plan on taking KTX to Busan and I could depart from Gwangmyeong station or Seoul station but what would be the best route to either of those from Incheon airport? And what time do you think I should book the train for to allow myself enough time to get through immigration, get cash/T-money card, and travel to the train station? I was thinking ~7pm but do you think that's enough time or cutting it close?

I plan to bring 1 big luggage (with smaller luggage packed inside) and 1 small carry-on bag and it'll only be me. Thanks in advance!!