1st class bachelor degree holders can opt to pursue PhD directly in most local public universities. the fees will be around the range of 2k for 1st semester and 1.8k for subsequent semesters.

rather than trying to come up with your own research topic with limited exposure to the current research activities in a that field (assuming u decide not to work first), it would be advisable to choose your supervisor instead. narrow it down to lecturers in your interested field of study with good research record (publications/patents) and of course the grants he/she has for that particular research.