Just before getting to the entrance to the AREX trains, there will be a convenience store nearby on the left side. I think its called a CU24 or something like that. They should sell the TMoney card there. I forget if the card is 2500 or 5000 krw. Once you have the card, you can load money onto it at a machine near the AREX gate. More info here.

The drive leaving ICN moves pretty good. Most of the ride is on the airport highway and it almost never has traffic. It is usually only after crossing the Han river does the traffic start to back up. But since Hongdae is not that much into the city, it will not be as bad as say going into downtown area or Gangnam. Although the actual street that Hongdae station is on can get really bad at times. Anyway, it is good to have options.

Enjoy Hongdae, especially on a Friday night in the area south east of exits 8 & 9 are very lively and fun. Not to mention all sorts of various cafes and eateries all around the neighborhood. A fun area to explore day and night.