shenzhen metro

How long does it take to travel from Longhua (Line 4 ) Schenzhen Metro to the end of the line at the Futian Checkpoint?. Do I need to change trains or is this a direct line?
Also on arrival at Futian, I am proceeding to Lok Ma Chau for further travel to hong Kong. Could you advise if I can purchase the Octopus card at Lok Ma Chau for travel to Hong Kong or do I just buy single tickets?
What is the most economical (there are three of us travelling)?