Not to be fussy . . . but "safety first" is pretty important.
I do not see in your sketch an "open" port or if you want a closed system, a high pressure / high temp. relief valve. At least one of those really need to be included.
I follow that you are just discussing concepts at this point, but I have done a small project like yours and it can create a surprising amount of heat, steam and pressure where you may be only expecting hot water.
Another design consideration (that will push you quickly into more heat and pressure issues . . . ) Since the smoke and dirty end of this whole thing is outdoors, you may wish to consider a "re-burn" barrel in the exhaust stack. They reburn barrels generate more heat than the basic fire barrel. If you routed your heat pick-up coils through or around the reburn barrel, you may pick up more heat than on the surface jacket of the burn barrel, as I understand your description.
You may also want to consider a thermostat disc on the exit pipe to regulate the pump flow -- i.e., when the exit pipe is at or above the design temperature, the pump flows, when the fire dies down the pump stops.