Hi I just went through several months of reviewing all the available keyboards on the market. I downloaded their respective manuals and spend hours in the music stores learning how to program them. I took into consideration the investment I was making and what happens to it in a few years. The last time I did this I ended up with a keyboard that performed wonderfully for 10 years with my rigorous travel and performance schedule of 4 to 8 concerts a week...I had to replace it because I wore it out.(posted 9174 days ago)The end result after months of study turned up that I bought a K2500 for its flexibility, sounds, available controllers, and the company support on the keyboard. It was the only keyboard that could fill the bill for what I need a keyboard to do when compared against at the other flagship synths/samplers.
Considering your classical background, with your goal to compose, my recommendation would be the K2500 with the optional Orchestra (and piano) board installed. The sounds are phenomenal, and the sequencer in the K2500 is easy to learn and will allow you to put together some great pieces. I would also suggest having sample ram installed and pram. Sample ram will allow you to load and play sampled instruments and sounds, so anything anyone has recorded you can play. The pram allows more memory to compose songs in the unit. I am not sure if you need the KDFX effects board, if you are working on classical arrangement with symphonic instruments, the stock reverbs should work fine. But if you decide that you need multiple effects you can add it later. The same goes for the sampler board, you most likely wouldnt need it if you are not planning to make your own sounds. The K2500 will play samples without the board as long as you have Sample ram added, and there is more than enough sampled instruments out there to keep you busy for a long time. I would also suggest getting a hard drive installed for sample storage.
The other units you listed are good units, but for composition the K2500 is the best. The K2500 is expandable and will grow with you, and Kurzweil keeps coming out with new stuff for it. There really isn't another manufacturer out there with a unit that has dedicated classical sounds. Most of the units have some token sounds, but not a whole bank of Rom combined with the power of the K2500 unit.
I would strongly suggest that you would try out a K2500 that has both the orchestral and piano board installed, and also with sample ram. Make sure they are installed on the unit you try out for a good 'feel' of what it can do. Sometimes in music stores, the on-the- floor units can tend to have no options installed which wont demonstrate the machine's dramatic capability and expandability to you.
Mark