Saya perlu buat keputusan sekarang.

Perlu is a modal verb in Malay, meaning “need to.” You can indeed swap it with harus, which also expresses necessity or obligation. The nuance is that harus can sound slightly stronger—closer to “must”—while perlu is a bit softer, like “need to.”
Example:

• Saya perlu buat keputusan sekarang. (I need to make a decision now.)
• Saya harus buat keputusan sekarang. (I must make a decision now.)

Yes, both are correct.

• Membuat keputusan is the fully prefixed, formal form (meN- + buat → membuat).
• Buat keputusan is colloquial and much more common in everyday speech. It literally means “do/make a decision.” In writing or formal contexts you’ll often see membuat keputusan, but in conversation you’ll usually hear buat keputusan.

No, it wouldn’t sound strange—it’s just more formal.

• Saya perlu membuat keputusan sekarang. is perfectly fine in writing, presentations or in polite speech.
• Saya perlu buat keputusan sekarang. is what you’d typically say among friends, colleagues or in casual settings.

In Malay the adverb sekarang (“now”) is quite flexible. Putting it at the end (…keputusan sekarang) emphasizes WHEN the decision must happen. You could also say:

• Sekarang saya perlu buat keputusan. (Now I need to make a decision.)
  Both are grammatically correct; you choose based on the flow or emphasis you want.

Malay often omits the pronoun if context is clear. So you can say:

• Perlu buat keputusan sekarang.
  It’s understood you mean “I.” However, dropping Saya makes it sound quite abrupt or like a note/reminder. In normal speech, you’ll usually keep Saya unless it’s very clear from context.

Yes. Memutuskan is the transitive verb “to decide.” You could say:

• Saya perlu memutuskan sekarang.
  In that case you’re literally saying “I need to decide now.” If you want to specify what you’re deciding on, add an object:
• Saya perlu memutuskan hal ini sekarang. (I need to decide this matter now.)
  By contrast, buat keputusan pairs the verb buat with the noun keputusan.

You can add juga to get sekarang juga, which translates to “right now” or “immediately.” For example:

• Saya perlu buat keputusan sekarang juga.
  This underlines that there’s no time to waste.