Don't take what I say as expert opinion, it's just the way that's been successful for me thus far.
I have ( simplified ) a system of both an open unpressurized heat storage tank, and a closed system with expansion tank, air bleeder, and secondary circulation pump. So I have had to plan for the worst of both worlds, but its not all really that complex.
At least its working ok circulation wise for me.
Are you planning a secondary circulation system, or do you plan to place the radiator loops in this as well, it could make a big difference.
In this system in the image if it used as it appears without the circulation moving through circuits that radiate heat,
I would place the pump in the lowest output of the open tank
The pump would then fill the water heater on it's (water heater's) lowest input or anything other than the top most out of the water heater. It might also be a plus for keeping air from building in the hot water heater if the filled water level of the large storage tank was higher than the water heater by at least some measure.
This kind of setup should minimize priming and air collection.
There is debate over if this loads the pump more because of the cooler water temp,
but then there is the argument that the pump will stay cooler so its windings are not as stressed.
I'm not an expert, I just go by whats working for me.