NO! HKSAR/SG passport holders can stay in South Korea for up to 90 days without a visa
Public transportation (both bus and subway) is great in Korea and takes you almost anywhere in the city! Transfers are also pretty easy. It's best to get a T-money card from a convenience store and charge in subway stations, etc. A helpful blog post here on T-money cards: https://blog.trazy.com/korea-subway-t-money-card-guide/ In terms of using a map, we recommend downloading the Naver/Kakao Map app on your phone, rather than using Google Maps. Naver/Kakao tend to give you a more accurate route. Kakao Taxi is the best app for calling taxis, but it's usually pretty easy to hail taxis on the spot too.
October in Korea is typically mild and pleasant, with daytime temperatures ranging from 15°C to 20°C . Light jackets or sweaters are recommended, especially for the cooler evenings.
Dial 112 for police and 119 for fire and medical emergencies.