Hi myktyl,
and welcome to the forum community!
In theory, SSD should increase the performance, and provide a more robust streaming and a better polyphony, as far as the drive is concerned, but in practice, we never did any actual tests, as these drives are still on the expensive side.
So it's very hard to make a recommendation here... but I agree that the new iMacs look very tempting.
I´d like to hear the opinion of others, of course - but this machine might be overkill for "just" the Special Edition. And you might be better off running your OS on the SSD drive, actually. But I´m not an expert in this area, I have to admit...
Regarding pianos: Whichever piano feels right for you is right. Take your time and check out which piano has the best feeling. Some pianos and masterkeyboards offer more options to manipulate factors like the Velocity curve, which might be helpful, but you can also change this in most software as well.
Hope that helps!
Best,
Paul
Paul Kopf Product Manager VSL