HK to Dongguan/Foshan

Hello there, I hope to fly to HK in April and I am going to Dongguan. Time allowing, I will go to Foshan too. How far in advance do I need to book either of those trains? Is it easy to book? Can I buy a ticket at the station? How many stations does HK have? Is Hung Hom station easy to get to? So many questions!

I have never been to HK before, is a guest house a good idea? I will possibly stay in HK for a day or two, is it an expensive place?

Thanks in advance for any assistance with that lot!

Peter, England.