Right - You need to get a proper audio interface if you are going to make any progress with mics and other sources. Look at the Scarlett 2i2 type devices.
You could record the audio output from your Kawai into the interface, and record your voice and job done - BUT, it's frankly, a bit silly. You could drive your car only in fourth gear, but that's not the sensible way to use the gears it has in the gearbox.
1. Can you play the music 100% reliably every time you play? If you make a mistake, you go back to the top and play it again. Next time you make a mistake somewhere else, so gradually you get tired and your first note error free performance is NOT the best one. If you are a pianist, you'll get this. So you record the MIDI data, and when you accidentally clipped that Ab, you can simply remove it. You can change the wrong note you played in the chorus with a mouse click. You can look at that note in the loud chord in the chorus and see why it was weak, and you can change the velocity of it from 29 up to 40 to match the others. Recording it direct offers none of these time saving features.
2. Can you improve on the Kawai's piano sound? To be honest, yes you can, if it is not suitable for the song. My Gem has a usable piano sound - quite realistic on stage, but if you listen carefully, you just know it's not real. Some of the sample and synth VSTs sound like a good grand, and importantly a good grand in an excellent room. That's enough for me.
3. If you have never even looked into cubase, then buying even the limited version is daft! Download a demo of all the ones you consider and try them. One will shine for you, but beware, making the wrong choice is difficult once you get good. All of the popular sequencers have very loyal fans. None are rubbish. They're just all a bit different. Swapping later takes real courage and patience, so important to get right. I started with Cubase 20+ years ago and I'm still with it. Also check that your computer is not only just within the specs on all these DAWs - if it needs X and you have that, just - it's NOT good enough.
Download some demos, get a proper audio interface if you don't have one, and start learning. You seem to be making decisions randomly. One moment ready to9 shell out £300+ for a full cubase system, next, going back to recording the keyboard's audio. This is just too random for long lived results. If you sing and play at the same time - you will have lots of editing to do, even if you are a diploma grade pianist. In fact, if you are, your editing gets even more critical.
If you look carefully, many of the interfaces come with free software, usually the cut down versions so you can try for free!