Transfer Beijing Capital Airport - Beijing South Station - Tianjin

I will be arriving BCA 12:15 (Oct 7, 2012), after clearing Immigration and Customs, plan to take a taxi to South Station, then buy a ticket for the bullet train to Tianjin.
(1) Is this a reasonable plan? My main concern is, are train tickets to Tianjin readily available without advance reservation, this time of year (on a Sunday)?
(2) Arriving at South Station, are there signs and directions in English to find the ticket counter with English speaking attendant?
Thank you.