Maths A and Maths B are parallel qualifications, there is no point doing both and which you choose is a matter of personal preference. Further Maths would be an optional extra. The same applies to the different English options. I suggest you have a good look at the information on Edexcel's website to try and decide.
If you wish to be a private candidate then all centres will do for you is provide a venue to take exams. You pay money to enter exams, they arrange the papers and the desk. It is entirely up to you what subjects and tier you choose.
I notice from Edexcel's website that Commerce is not available to UK students after 2014, but that implies that it will continue to be available to overseas students.