1. Actually you will need someone to get you books from Pakistan, as most of the books used for CSS are written locally and are not published internationally. But that is not a big issue. First you need to finalize your optional subjects.

2. You do need some advice from Pakistan for the CSS exam, as there are some differences in foreign education and Pakistani education.

3. There are 6 compulsory subjects which everyone has to undertake. They carry 600 marks. You have to choose optional subjects carrying 600 marks. There is a long list of subjects available to choose from. Some carry 200 marks, some carry 100 marks.

A brief introduction of the syllabus and subjects is given in the link below.
Syllabus

Keep in mind, that since this exam is the premier exam of Pakistan, so you need to keep in touch with Pakistani newspapers, Pakistani news channels and perspectives of Pakistani people. The syllabus contains international author's books for every subject but you have to rely heavily on Pakistani local books as they will get you prepared for the exam reasonably quick and easy.