Any idea how to purchase the 1 way bus from the peak to central station?

The ticket fees must be paid on the bus, so you cannot buy the tickets on the peak. The childern below 12 years old enjoy half of the ticket price. The adult ticket price is 4.2HK$.
If you want to take the 15C, the bus will take you to the Central Bus Terminal, which is very close to the MTR Central Station and you can definitely walk to the station.