I have an eberspacher D2 and a much older B4 unit (both air heaters, not water)
On the older B4, there's 4 parts - the burner, the control unit, thermostat, and pump. The pump is just a generic diaphragm pump, the thermostat just a thermostat, but the control unit and the burner itself are eberspacher only.
On the newer D2 unit, there are 3 parts - the burner, which has the control inside it, the pump, which is an eberspacher only part, and the 'interface box' part, with a little LCD on it. The 'interface box' is nothing special though - its an eberspacher part, but it could just be replaced with a switch or a stock thermostat, but the other parts are required / eberspacher only.
This may give some minor insight to whats needed on the similar units.