Hello! Could you please help me with my travel plan below?

We of 2 people would like to purchase either Z-train or T-train from "Kashger" to "Guangzhou" which will be leaving in the evening on weekdays during the late of January 2010. And we have already made the decision to select "Soft Sleeper" for a compartment eventually. So here our questions are:-
1.) How many days do we have to buy those tickets in advance successfully?
2.) Is it necessary to buy those tickets by ourselves? Or we could have our relatives there bought for us.
3.) Will it be crowd during those days?
Thank You very much for your appreciable helpful information