For what my opinion is worth, digital pianos in the last 10 years have made a lot of effort to reproduce the feel of a real piano. However nowadays, the real leader seems to be Kawai and their CA-series. I have my eye on the CA-67, it uses the Shigeru Kawai Concert grand samples, and their keys are real wood, real hammer action (not simulated), and feel just like a piano.
Now your ability to shape melodic lines is directly dependent on key action. I have a craptacular Casio AP-620 which is pretty much broken, but the one thing I like about it is it has soft keys that are "close" to a maintained piano. You really do benefit from lighter keys, but not the plastic super light ones that don't feel like a piano. Heavy keys tend to limit your lower pianissimo range drastically. Not good for expressive playing.
If it's in your price range, please consider the Kawai CA-series. Their tech has far surpassed Yamaha in the past decade, and you will not be sorry you got one. Please feel free to research the line, they are a fantastic product. I have researched a lot of modern digital pianos and the consensus seems to be, no-brainer Kawai CA-series (if affordable for you).
Everything you said is of legitimate concern, but I am of the mind that if the music is put at the forefront of your mind, the pianist must learn to adjust his/her playing based on the action of the piano they have at the time. This requires a keen ear, but the most beneficial (and thus most costly) route would be to pick up something that closely resembles a piano as I suggested above. It's silent, but get a good set of headphones, there's no point in listening to beautiful sound samples through crap headphones. Hope this helps.