Tankless Water heater Ground Water Temp | Terry Love Plumbing Advice & Remodel DIY & Professional Forum

I want to install a natural gas tankless water heater, converting from a standard tank type. My house is 2 story 3200 square feet with 2 occupants. My ground water temp is 42F. I live in a cold climate in North West Montana, I am anxious to get the right system, any recommendations or concerns? I was looking at Rinnai tankless systems.