The infinity control (like the unit itself) is very confusing and not very user friendly..........Before you spend all that money, you may want to see one in action somehow............it could save you alot of headaches down the road.
We use Infinity controls regularly. Like any other digital thermostat, there is a short learning curve that reading the manual cures quickly.
Your contractor however is both right and wrong! The Infinity is designed and works best, when used as a "System" not just one or two components. In this case, your furnace will work with ANY thermostat, but it will not allow you to have all the functionality of the Infinity System.
The unit is designed to communicate. The control, is more or less a small computer that monitors and adjusts the furnace blower to meet the demands of your existing indoor climate, based on feedback it receives from sensors built into the system.
Duct pressures are monitored - If the filter is dirty, you are notified on the tstat. More importantly, if the pressure indicates a frozen evaporator coil, the unit will shut itself down to avoid compressor burnout etc.
So... Yes, the Infinity gas furnace will work with you HW thermostat, but you will lose of the important features of the Infinity unit. You are spending quite a bit on the unit already, its worth having the Infinity control!