Scott -
You shouldn't size a pump based on horsepower.
In order to do it properly, you will need to do some calculations:
Determine the BTU/hr you need to transfer to heat your house
Size the heat exchanger to meet this need
Determine the anticipated temperature drop across your heat exchanger
Calculate the flow rate of water needed
Calculate the friction loss in your piping based on the flow needed (Very important not to skip this step !!)
After you have done all of this then you can pull out the pump catalog of your choice and find a pump that meets your friction loss and flow requirements.
All this said and done, a Hardy H2 OWB ships with a Taco 009 which is 1/8 Hp. Other manufacturers seem to have there own "standard" size pumps that will meet most piping / heat exchanger setups.