Assuming you have no heater now, the numbers seem totally messed up. If the gas heater requires that a natural gas line be run from the house then it is way low. The heat pump number seems way high even if it includes running 220VAC electric. I think you should get some other bids.
As for other modifications they are minimal. Obviously you will have to cut into the return piping and run it to the heater and then connect the heater output to the return lines. You might want to install valving so that you can by-pass the heater at times. Other than that you should be fine as is, unless your pump is at its limits in which case the extra pressure drop from the heater might restrict your flow. You can always install it and then see if you need to upsize the pump based on actual flow rates once you are done.
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