What bus from Beijing goes to Datong and from where? What is the cost and travel time?

You can take the direct coach from Liuliqiao Bus Station. The bus schedules are from 07:10 to 18:00. It costs CNY 128 or CNY 133 per person. In general, the duration is about 5 hours.

Can I buy the bus ticket in advance or do I have to pay on the bus?

You can book the bus ticket at the official website of Beijing Interprovincial Passenger Transport System. Since the booking system is in Chinese, you may ask your Chinese friends for help. Or, just buy the ticket at the ticketing window of the bus station on the spot. Don't worry! :)