Will train ticket to Xian be sold out for this date and time?

Train Z-19 or T-43 from Beijing West to Xi'an. Thursday October 13, 2011. Where in terminal 3 of Beijing Capital Airport can tickets be bought? I am finding conflicting times of departure for these trains. What are the true times? How long does it take to get from the airport to Beijing West by bus line 7? by shuttle train and subway? I will be leaving the airport at about 18:00. Consider traffic at this time.