I haven't seen any recommendations for the Korg D1 - it must be the cheapest price you can pay for getting a top-tier action.
For the same money as a Roland FP30 you get the same action as in the Korg Kronos 88 (a 3000 euros workstation/stage piano). I have tried it and liked it.
The only thing to mention is that the grading is in 4 zones if I'm not mistaken (as opposed to 7 zones in the Roland FP90 for instance, to which this action compares well), but I didn't perceive it and it didn't bother me while playing.

I am using the KORG D1 with Pianoteq Pro thru a Steinberg UR22C which runs the whole thing with no audible latency - everything is just perfect! I am exceedingly happy with this set up.
This keybed is absolutely fantastic, highly recommended. Not sure you can get anything better for the buck.
........ . . . . .   .   .   .   .
Except one thing :-(
I get a glitch now and then from my laptop, a Dell XPS 15 i7 - 7700.
I tried to deactivate the acpi and a lot of other things as recommended by people that seem to have solved the glitch issues. At no avail. Ultimately I will have to use the Latencymon some time soon to find the mysterious culprit!

Osho

I bought the Nord Piano 3.  It doesn't play well with Pianoteq.  Let me quickly summarize:

1) their fancy 3 sensor velocity calibration does not apply to MIDI out, only their internal patches.  So the sound is uneven and you really have to mess with Pianoteq to get a curve that works.

2) None of the MIDI controls on the unit pass through. You cant Transpose, for example, or octave shift, or split or anything like that.  The only thing that goes by MIDI is the note and the Sustain pedal. 

3) They keys are noisy. Really noisy. My neighbor complained and pounded just from the flutter sound! Man.. after paying that kind of cash you don't want that to happen to you.

So buyer beware an read carefully.. Nord sells a super high end version of the 3 for over 3k that apparently does pass midi control, but look at that value added price.. Yuck.

I'm giving the Nord Piano 3 to my dad and picking up the Kawai VPC1.. Quiet keys, software that I can transpose with, and $1000 cheaper.

Hope that helps somebody.

same impression for me, I only use  the dedicated curve of the  VPC1 and the "normal keyboard" and "moderately fast keyboard" in the pianoteq presets

[...] customizable velocity curves (the numa can create 'self-adjusting' presets based on sampling your actual style of play).

Interesting, could you please write a few words, how it works?
Does it produce satisfying results?
Pianoteq's Keyboard calibration assistant for example looks nice on paper, but I can't get adequate results with it. As if something important is forgotten in that algorithm.

Main midi keyboards are a circa 1991 Peavey DPM C8, and a 2000's Studiologic Numa white. Both are 88 key, with global aftertouch, and customizable velocity curves (the numa can create 'self-adjusting' presets based on sampling your actual style of play). I have an Axiom 25 for synth and organ (and to provide pitch and mod wheels for a numa white)...

I don't own a midi controller that actually generates sound. Pianoteq continues to be my goto piano sound, even though I have Steinberg's 'The Grand 3', and other in-the-box sampled instruments. In the synth world, for perspective - Pianoteq competes with my Prophet 12 module, and Waldorf Streichfett... august company, and it holds up nicely!

There seem to be many options and decent keyboards around, everybody has his own favourite.

How about Nord keyboards that many pop-music and jazz players use?

And about used, second-hand (cheaper) options that are still worth considering and durable?

Technically, are there different types of MIDI or is it all the same?

Thanks again, best wishes Michiel

Am considering buying one myself till I can afford the Casio GP500

Warmest Regards,

Chris

P.s.

I've tried many, many different actions and for the money the CDP action is the best and it also integrates well with Pianoteq

My personal preference is for the action on the FP30, which I own. The Yamaha is notable because it supports both MIDI and Audio over USB.

I purchased a Casio AP-270 (budget piano) with hifi speakers on top of it (Focal chorus 706), both connected to an old computer and hifi amplifier.
Adding the cost of Pianoteq and audio cables I paid just about 1000€ (I had the computer and the hifi amp already) and I'm really happy with the result.

In my experience, adjusting Pianoteq's velocity curve and dynamics slider had a huge influence on the realism of the playing experience.
I would advise to spend a lot of time tweaking Pianoteq to fit your piano and speakers perfectly, and do the same thing for your students.

The quality of the sound is also a big factor. Not many budget digital pianos have an "aux in" input to receive audio from the computer and most of them do not have good speakers anyway. Good external speakers and/or good headphones can help a lot to provide a realistic response to your playing.