Hi Shane and welcome,
As a bit of background information, the size of any heater or heat exchanger system will depend on the pond temperature that you wish to maintain and taking into account the heat losses. A good rule of thumb is that 1 kW per 1,000 gallons is sufficient to heat the average pond to the mid twenties in summer and will easily maintain a temperature above 10°C in a covered pond in winter although it might struggle to get much higher than that in an exposed pond during a harsh winter.
Many people associate kilowatts with electric heaters but they are also used to assess the outputs of other heaters such as gas boilers.
On your pond, the boiler you've been offered works out to 3.8 kW per 1,000 gallons and the heat exchanger will be capable of transferring heat from a boiler at a rate of up to 10 kW per 1,000 gallons so the pair together will be plenty. In fact, so that when the pond temperature begins to drop and the boiler switches on and begins to raise it back to the set point, it doesn't raise it too quickly, I'd suggest that you install a bypass around the heat exchanger.
This is a little bit fussy if the boiler is always ready to switch on and the stat can control the temperature to about one degree centigrade but is more important during initial heating or any other reason why the heat source isn't available for an extended period and the pond temperature drops significantly. It's also a more efficient way of heating water if the temperature of the water coming out of a heat exchanger can be controlled so that it's between 1°C to 2°C warmer than the temperature going in to it.
There are two ways to do this; the first picture shows the way that I most often see a bypass installed but the second picture shows a method that will give more control over the rate of rise. Using typical figures, I once calculated that the first method can only control the flow through a heater from 100% down to around 70% while the second method can control the flow through a heater from 100% down to around 20%.
This method gives less control over the flow through a heater or heat exchanger because most of the flow will take the easiest path straight through the heater and will "bypass" the bypass. The typical control range through the heater is from 100% with the valve fully closed, going down to around 70% with it fully open.
This method gives greater control over the flow through a heater or heat exchanger because, with the valve fully open, most of the flow will bypass the heater and, as the valve is closed, more and more of the water has to go via it. The typical control range through the heater is from 100% with the valve fully closed, going down to around 20% with it fully open.