airport bus from Beijing west Railway station

hello I am asking a question about getting the airport bus from Beijing West railway .
I realize i need to come out at the SOUTH entrance.
Is the Bus stop close to the exit ?
Can i buy ticket from driver, IF not ,where do i buy my ticket and how much?
MY train does not arrive into Beijing west railway station until 8.20 pm on Sunday 7th Oct .What time do the buses run until ,or should i try to change my booking to an earlier trainbefore the tickets are issued