Well, first of all keep a few things in mind:
a. It does not matter what background you have and trust me being an Engineer does not give you any sort of edge or advantage (I am an engineer as well and I know this from experience).
b. DO NOT take anyone's word for anything. Be your own man. Let NO ONE influence your choice of optional subjects.
c. Choose subjects only after reading their syllabus and analyzing them. If you feel you are interested in any subject and feel that it's in your comfort zone...then go for it.
d. People will talk about scoring trends. All that is relevant to an extent but not final nor conclusive. Your hard work is all the scoring trend and scoring prediction you need.
e. Be 100% certain about any subject before opting it. Make up your mind and stick with it. Pivoting will only hurt your preparation.


As far as material is concerned, the syllabus posted by FPSC will suffice. Majority of the books are either downloadable or available in market. Search for them and also use the Internet extensively. Use YouTube as well. DO NOT use any summarized books or take notes from any one. Make your own notes. You are a professional that is why I am asking you to clearly realize that CSS requires time and hard work. Therefore, if you want to pursue it you should do it like you mean it. Make a proper time table (Very Important) and stick to it. Be very very PATIENT. Be CONSISTENT. Be SMART about how you approach all your subjects. You will be offered shortcuts and what not but you will pass only if you DO YOUR OWN WORK. For MCQ's you will find this forum very helpful and you can also search the internet for MCQ's or purchase books if you like.

Remember: MAKE YOUR OWN DECISIONS. LET NO ONE INFLUENCE YOU.