hi vio,
the module version of the sd5 is released soon, why not use that and add, either a controller keyboard, or a dedicated piano keyboard, example the new casio series are pretty good, as i the kurzweil pc2x (it has 76 graded hammer action too? and under 20 kgs)....good thing about the sd5 module is you already know the op system....same as the sd with a few more "bells and whistles"
cheers
dennis