I can't offer specific information but I would look at www.worknplay.co.kr
which has forums on housing and a number of other things to get u going etc. Personally I would NOT sign a lease while or choose a house while abroad. What you are told and what you get can often be vastly different in Seoul. Heaps of apartments around, and now winter is over people are moving around again changing apartments etc. My suggestion would be to set up some contacts to look at places when u arrive, but stay in a hotel for 3-4 days until u get what u want. 40,000-50,000 a night and comfortable, but will save u a lot in the long run.