You've figured out that throwing random parts at it is more expensive than having it diagnosed by a mechanic. The thermostat is the last thing to consider. If the heater is blowing cool air, have the head gasket checked for leakage. Combustion gases pooling under the thermostat will keep it from opening. They need to be hit with hot liquid to open. You will also feel the upper radiator hose is not real hot. If it IS too hot to hold onto for very long, check if the fan is turning on. If it is not, you must determine if the Engine Computer is not turning on the fan relay or if there's an electrical problem in the relay circuit. When the fan doesn't turn on the heater will blow very hot air, and it will not overheat while driving at highway speed.