I'd say 16gb is enough, even for a long run usage. But some might disagree with me, and suggest to opt for 64gb instead.
Hundred songs with plenty of social media usage is considered moderate usage. Even if you keep songs in FLAC, should be fine.
For my last usage, I used Spotify for music, and keep the best photo of specific event to be kept in phone. Installed few games, still got plenty of storage (8gb+ free) after few months of usage.

RM150 per month can go a lots, I don't think he need all the privilege of postpaid150 during his study.
It's the best to observe and get used Iphone SE, which I did last time. Grab when you find good deal with local warranty instead of paying full premium or rebated price with contract.

My first contract with Digi88 for Galaxy Note 2 is really painful during student era for 2 years contract. Which later I downgrade to Digi50 and Iphone 6 with 1 year contract.

I think better stick with prepaid and get used SE unit. Get postpaid if you need it.
SE is cheap already, value for money.

I'd prefer get used unit with local warranty and the same discounted price without any contract. You may choose otherwise.

Plan with phone and without phone are never same, so basically you're paying more (inside postpaid monthly bill) for your phone. While you get more value for the same/similar monthly pospaid plan.

If overall deal for postpaid with phone suit your need, might be good to grab the offer. Otherwise, stick with prepaid and get the phone outright.