replacement HW heater and air handler

[ADB]

I currently have a Takagi tankless water heater hooked up to an Apollo type air handler. The system provides heat and domestic hot water. The water heater has a crack in it from cold weather (it's in the attic and the heat was turned off) and the coil in the air handler is bad. So both units need to be replaced. So I am looking for a contractor to do that at the least possible cost (some of the quotes I got are quite high). Also, where does the "heat exchanger" come into this?