Could South Korea repeal a North Korean attack?

I think they could, yes. It would be a bloody awful war, but the South Korean military (although half the size of the North Korean one) is a relatively modern and well trained force. To give an example, the South Koreans have collaborated with the South Africans in designing a third generation main battle tank, the K2, that will be fielded soon. It should be roughly equal to the U.S. M1A1 Battle Tank. The North Koreans most modern tank is an updated Russian model from the 1950's.