One thing to be aware of is that, while the WK-7500 does have sliders that simulate drawbar adjustments, they can only be set to 4 positions, rather than 9 positions as on a real Hammond or clonewheel.
SInce you already have the WK3700, and it is doing the job for you except for organ, then for not too much more than the price of the WK-7500 you could get an iPad (Apple has 1st Gen refurbs for $399, if you want to keep the price down), GarageBand ($5), and MIDI interface ($70), and run a pretty nice sounding organ out of that, triggering it from your existing board. It has its limitations too, but I think it's impressive for the price, and the iPad opens up a bunch of other possibilities for you as well. And if you happen to already own one, well, then this is a dirt cheap way to go.
Maybe this is the best place for a shameless plug! Our new video at https://www.youtube.com/watch?v=3ZRC3b4p4EI
is a 40 minute adaptation of T. S. Eliot's "Prufrock" - hope you check it out!