Ok. Thanks to everybody. I heard the longest hesitation would be panel planning (actually, I changed n times my mind...), but nobody told me of electrical architecture perplexity.
I started with Z-12. Then, when I realized that SD-20 needed a battery (so, in case of battery short or battery contactor failure, no juice), I abandoned it for Z-11. It has an endurance bus, so one can simply switch battery off and continue fly with MEL. Then, I thaught: I can always add a second smaller battery. But...why not go with Z-14 with an aux. non cranking battery. since I'm planning from scratch?
When I started considering Z-14, I was enthusiastic. Then, I understood that I couldn't probably make both alternators (L-40 and SD-20) work together; I turned down thinking using the main one with crossfield contactor closed; that I realized that having an aux. smaller battery will probably make problems in charging (since the different ampereage of the two batteries); then, when counting weight, I realized that adding a second Odyssey PC-680 would be too much...for a RV9. Maybe for a RV10 it would be the best choice.
Finally, I'm persuading myself on Z-13/8. SD-8 costs less and...weights less. It goes after battery contactor and let me save 1 amp. battery contactor coil current in case of main alt. failure. And now...it may work without a battery.
I ended as Bob predicted in his book. Time lost?
Camillo