Usually, travelers could purchase the train ticket from Shanghai to Guangzhou by themselves at Shanghai Railway Station in advance. Since you are not in China, it is suggested that you search for a travel agency online supplying ticket booking. The luggage limitation is the same as flight's: 20kgs luggage for free. The immgration will not take you too much time, and safe enough So, please take it easy! Actually, if possible, it is suggested that you fly to Hk from Shanghai directly. In travel low season, the airfare is so cheap and you could book a ticket easily online.