Whichi built-in engine is same level or close to default Pianoteq ?

You are going to have to flesh out your questions for any users that might be able to help to understand your question/problem rather than second guessing!

Some digital pianos have good sound -- Clavinovas, the Casio GP 510 (not sure if that number is exactly right). I was very happy with the sound of my Roland RP501, which isn't even a top-of-the-line digital piano.  And they all have a variety of different instruments, of varying quality.  But you won't be able to match what Pianoteq can do.  Pianoteq is a separate universe.  Now that I have it, I could never go back, and I'm not especially "techie." I don't do all the things others on the Forum do.  I just like having these beautiful instruments to play. You won't have access to the range of instruments or the ability to make the kind of modifications you can on Pianoteq. And none of the digital pianos have the kind of responsiveness that Pianoteq does.

One thing I noticed when I started using Pianoteq is that my touch was VERY uneven.  I didn't notice it on the Roland's native sounds, because the sensitivity to my playing wasn't there.  But with Pianoteq, the notes I was playing weren't even in loudness.  I've needed to work on building precision/control.  I'm still very much a beginner, having taken lessons in elementary school and returning after 60 years of not having touched a piano.  But I want to play for the rest of my life, and I want to do so using Pianoteq.  It's an essential part of my journey, and for me, there are no substitutes.  I don't want a better piano, just what I have, with Pianoteq.

Hugh Sung wrote a very insightful comment here on the site explaining the difference.  He is a concert pianist and is also the vice president of Cunningham Piano, so he sells Yamaha Clavinovas.  Here's the link to his comment:  https://www.modartt.com/references#sung

I have to say -- I don't know why you are having software problems.  I have Pianoteq installed on a laptop that I keep connected to my Roland.  I never disconnect it.  I just open the laptop, tap it to "wake it up," select the instrument I want to use, turn my piano on, and start playing.  Nothing ever goes wrong.  If I got involved in more intricate things there would be a learning curve.  But for me -- it's plug and play.  The only modifications I make are to turn off the pedal noises, which irritate me.

Much more knowledgeable choices than what I wrote!  There are a lot of digitals with "good" sound, depending on individual taste, but none do what Pianoteq does in terms of reproducing authentic piano subtleties, per what Hugh Sung wrote.  Even at my level, I've experienced a profound difference.

That's true.  I was totally thrilled with my Roland and didn't think I'd ever want anything else.  I was introduced to Pianoteq by a sound engineer who was using some of the same learning platforms that I was.  I was totally resistant to the idea, because I thought it was overkill for me (and it probably is, but I love it).  I listened to the audio samples on the website and was swept away.  Well, the rest is history.  I even had my tech guy (who has helped me for many years with various tech things) come in and set it up for me.  He guided me on the laptop to buy.  I'm only using it for Pianoteq.

So -- it definitely isn't something I "needed," and it was a significant splurge, but it isn't illegal, immoral, or fattening, and I really wanted it.  For Doremi, a better digital piano could be perfect.  The Yamahas (P515 and the Clavinovas) have the Bosendorfer and the CFX, which are both glorious.  And the other pianos listed in this thread are all very good.  I think that having a console digital is a significant step up from a slab, if that works for Doremi.  The acoustics are better because of the resonance of the cabinet, and it's more comfortable to use.

Hugh Sung is a concert pianist -- I'm not -- but Pianoteq gives me much more of the real experience of piano playing and I'm going to improve as I move forward, so it will mean even more to me.  I may or may not do more elaborate things with the technology.  I want to be able to bring the music I love to life, and I have everything I need to do that.

All this is so individual. BTW... Roland pianos are modeled, not sampled, and I do prefer that.

I considered buying a HP704 which is also modeled.
So you use a dedicated pc. I use a less dedicated pc to save space and money (Garritan is heavy resource requirements).
There's often a change in the OS which requires fixing because there's so much software involved.

Raspberry pi I would suggest is for people who are very comfortable indeed using computers. People who like tinkering behind the scenes. Alternatively it's for people who know someone like that that can do all that bit for them.

I was coming from the angle that you were probably finding computers difficult to understand. That perhaps you were older and were frustrated with personal difficulties. You shouldn't be offended as you have given us so little information so I'm having to read between the lines.

The Raspberry Pi solution certainly would not suit somebody like that.

As far as I understand it Raspberry pi are still under powered and right on the limit of being able to handle current Pianoteq, with polyphony having to be reduced for instance. So they might be only one major update away from not being compatible with advanced features or quality if the CPU requirements go up noticeably.

So for instance if they are right on the limit of handling Pianoteq then a Raspberry pi owner might have to halve their polyphony with a big update. Depending on their playing style and skill level this may be unacceptable.

If you wanted to geek out when chip shortages end then you can keep changing the guts of the little computer for a more powerful Raspberry pi, but then you're back at computer hardware again. Albeit small lightweight affordable and fun (for some) experiments.

Quite a low price. I haven't tried such a system myself or the Raspberry pi, therefore in terms of stability I don't know what the issues are. You're better off talking to somebody who has built a Raspberry pi Pianoteq system and perhaps someone that has made comparisons with a low end NUC.
Certainly an i5 version is going to have some extra software upgrade potential.
I would want to keep a few software updates in mind, unless you really are prepared for a fit and forget solution, as if you are a hardware manufacturer?