The SL-990's and VMK models all have the same excellent keybed, I assume the discontinued 880 does also.

For use with Pianoteq, NONE of the features matter. Pianoteq has its own velocity curve setting, volume, and EQ. All you need is a good keybed that sends keypress signals on the default  midi channel one.

The SL-990xp is incredibly homely, pictures don't do it justice. I put a nice black lacquer finish on mine and it's now quite lovely. It comes with a cheap foot switch for a damper pedal.  Throw it away and get the inexpensive MAudio pedal . Half pedaling isn't supported on any of the models you named.

You'll probably need a midi-to-USB converter. One of the cheap stub types will work if you already have USB extension cable. A decent 2.1 speaker system and you are good to go.

Oh, these boards are 50+ LBS, you need a sturdy stand, not one of the cheap X type stands.

I've never played the piano but I'm getting Pianoteq and a Midi keyboard for my 8 year old kid. 

I have a choice between the older SL-880pro or the newer SL-990xp, or the likewise new SL-990pro.

What I've gathered so far is this:

The SL-990XP has a single velocity, while the SL-880pro and SL-990pro both have 8 preset velocity curves.

The SL-990XP and SL-880pro have both sustain and volume pedal input, while the SL-990 pro only has sustain input.

The SL-880pro has aftertouch which I gather is undesirable if one wants more of a piano keybed feel.

Any recommendation?

And what about foot pedal control?

Thanks