You won't, and will never, be able to use that handset in Korea. They simply don't have 2G GSM services.

Additionally, my understanding is that given the current law, you'd never be able to buy a SIM and just put it into a new (unlocked, 3G) handset in any case. You need to buy a service plan and register your phone with the authorities. Only whitelisted phones can be used in Korea with Korean plans.

So, your choices are:
- Buy a handset (can probably get a so-called "card phone" second hand from Itaewon)
- Rent a handset with its own service plan and pay the bill when you leave Korea
- Rent a handset that takes your own SIM, and roam
- Buy a 3G handset (from home) and roam