Ok so the main question is related to size and scale. How much heat do you plan on extracting from the ground? The higher the demand, the more water you will need. If you are planning on using a tank and coil heat exchanger, your cyclic demand will most likely dictate the size of your tank (unless you have a river flowing rapidly through the tank). Cooler entering water, lower flow rate, more demand for heat, all of these equate to more surface area and a larger tank volume.
Next question is: freezing or no freezing? With your big flood bed, and perpetual flow of water, a submerged dx loop or conventional glycol or brine ground loop and a brazed plate heat exchanger could bounce against zero Celsius and provide a literal wall of heat. At the phase change temperature, there is a boat load of heat to be had at constant temperature.
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