Here's the rundown with the M3 in Korea. I actually just shipped my car off today (going to Virginia/DC) since USFK screwed me over and I lost my job. Finally heading back to the US after 7 years.
There are two main BMW dealers, Kolon Motors and Hansung (I think) but Kolon Motors is by far the biggest dealer. They have service centers all over Seoul. If you get stationed in Osan, I think Suwon is your best bet but I'm not sure; I've always used Seoul service centers.
In Seoul, the best maintenance center is near the World Cup Stadium in Mapo (still Seoul). That is their main service center. A friend of mine who works for BMW Asia says it's the best too.
You'll only have a 2 year warranty and that only covers manufacture defects and what not; you'll have to pay for your own oil change/rotors/pads/etc. Generally speaking, parts are much more expensive so it's best to order what you need by mail. Having an APO box helps. Same goes with tires; I used to always order via Tirerack. Hankook tires are made here in Korea but they're twice what you would pay from the US. Labor is relatively cheap though so buying your own parts is really the best thing.
I also recommend getting a blackbox camera (you can get good ones here) in the event that you get into an accident and some guy tries to scam you. I hate to say it but when it's a foreigner's word versus a Korean's, police will usually side against a foreigner.
Besides all that, driving is great in Korea. Roads are pretty awesome (highways) and no cops on the road to catch you speeding. Also, the nicer car you have, Koreans have a tendency to give you a wide berth since they don't want to be liable for any damages.
If you need help, there is a Kolon Motors office right outside Yongsan and Osan and you can probably go through them to set up any kind of appointments with a service center.
Good luck and have fun here in the RoK!