the Name +귀하
It' a lot better to call the name with their postion if it is for business.
'홍길동 부장님 귀하,' / '김갑돌 사장님 귀하,'
If you don't know their postion,you could use '씨' or '님'
'홍길동 씨 귀하/'홍길동 님 귀하 (님 sounds better to me.)
For the ending, I think in Korean culture there is not a fixed format equvalent to Sincerely in Einglish.
I would say 감사합니다. / 그럼 빠른 회신 기다리겠습니다./ 잘 부탁드립니다. etc
I think it depends on the context of your letter. But It is better you don't use '수고하세요.', which is usually used by the elderly to the younger.