oleg7, you presented a very good alternative to a completely software based arranger system. This can give you many choices in sound enhancements.

Another alternative might be to start with a workstation, e.g., Motif ES and use a laptop with a software based arranger, e.g., OMB or Live - Styler. The control surface may not be as good but it may support some of your other objectives (sounds, quality of keyboard, studio).

There are, of course, the very expensive approaches that may work for some, e.g., OpenLabs, Wersi, etc.

So as can be seen there are as many approaches as there are people. This makes it all very exciting. I have choosen to go the fully software based approach as a matter of principle and to prove that it is viable.