Hello Mr.Tinaaz
It is more than 150km and it takes about 70 minutes to get Guangzhou from Shenzhen by train ,the ticket is 75 RMB ;You had better don’t off at the Guangzhou East Railway Station at the midway , because it is quite far from the Canton Fair .You can buy the ticket for the Guangzhou Railway Station (general station) .There are two Canton Fair in Guangzhou ,the old one and the new one ,the old one is not far from the Guangzhou Railway Station and you can take the subway line 2 and off at the next station –Yuexie Park or you can go there on foot .If you plan to go to the new one ,you can take the subway line 2 and off at the station named Pazhou(琶洲) .

Good luck!