We have searched and found the right keyboard controller for the HX3 MIDI Expander: The M-Audio Code61 offers a decent keybed, the obligatory nine faders for drawbars and many other well-programmable controls. Our templates for controlling the HX35 are available for download. Our label set for labelling the controls completes the HX3 organ for less than $1000.
Prices (for US customers, minus German VAT):
HX3 MIDI Expander: $685
Label set for Code61: $12,70
Update: The M-Audio Code61 is apparently no longer in production and is no longer available at retailers. But you can still find a lot of used equipment, for example on Ebay. This makes the setup even more affordable.