How to go to Beijing South Railway Station from Beijing Airport?

will be traveling from usa little heavy luggage to Beijing Airport then planing to go to Tianjin : what is best way to reach to beijing south train station to get bullet train for Tianjin.....Does Bus no. 3 goes directly from Bejing airport to south station : would i be able to store heavy luggage in bus....how many Km and how much time to south station....is it direct bus or i hv to change to subway ?? or i would be better of taking taxi from beijing airport to beijing south train station. how much taxi cost and how long it will take.for Bus where do i buy ticket...do i need exact change....your advise/suggestions would be highly appreciated. Thanks for your help or any other suggestion due to heavy luggage. what time last train from south station to tianjin.