Best choice for Digital Keyboard Arranger ~($400 - $700)?

My son is a Rapper/Songwriter. He's getting better and better and now would like to have the ability to create better music above what Garage Band can do. I have found various options, but would like some input. We would like keep it under $500 since he's still a beginner and we have a budget.

This is what I've found. Would love your feedback, thoughts and other suggestions. The Korg MicroArranger has gotten our attention and is the right price point. But if something for less gets you the same or if there's something a bit more that's way better, we want to know.

Yamaha Piaggero NP-V80     $399

Casio WK-7500:             $449

Korg MicroArranger:             $499
Korg Kross-61                     $699

Roland E-09:                     $599

Feel free to send other suggestions.

Thanks!