I was an international school student myself 2 years ago, so I know where you are coming from. I would recommend you take the SPM syllabus if you are going to venture into med school or any government sector. Taking merely a single BM subject would not satisfy the government requirement to enter into med school, that's how my friend was forced to retake the SPM alongside Sejarah to properly get a cert. Even if you are not going into those jobs, as a Malaysian, it is strongly recommended to take a Malay language exam, SPM, if possible.
To get a cert for SPM, you will need to pass 2 subjects:
1. BM
2. Sejarah (Not too hard)
As for IGCSE BM, I had also taken it during my time because it was considered a free A* for most Malaysians, this will help you in securing scholarships in college.
I took both the SPM and IGCSE, so reach out if you need more help