What image scale do you want, as that will dictate focal length. As per Ray's thread from a few weeks ago, sensitivity is related to pixel size (fixed now for you at 7.4 um) and f-ratio. So then, considering those factors (image scale and f-ratio) together with the load limits of your mount, will help narrow down the choices.
ps for imaging, i'd caution against loading an EQ6 all the way up to the 20kg limit. My own current deep-sky setup (modified VC200L with ST10XE, filterwheel, robofocus and extra guidescope) would be about 14kg, and I reckon that is about as much as i'd want to go.