2000 Mercury Sable no heat & overheating

Hi
The smell and window icing tell me heater core leaking. If coolant is low that would cause your overheating. Assume it is cold where you live. Do you see white smoke a water comming from tail pipe? Doea oil look normal or a little like chocolate malt? Yes to either question head gasket failure.
Let me know
Merry Christmas

Let me add some more details----I have replaced the radiator and the thermostat because I bought the car and the radiator was rusted and leaking I also had to replace the thermostat due to rust----the mechanic I had look at it told me the heads are bad but I the oil is not milky and the car doesnt overheat all the time only when the car is at a stop and when you are in slow traffic and not moving over 5mph I went somewhere else a second opinion told me that the cause of the overheating was from the heater core being out, and everything else was fromt hat also---symthoms the car is overheating, when you turn the heat on no heat and it smells like antifreeze, it is winter and of course my inside windows iced up and there is leaking under the passenger side dash it is not bad just a little. If it is the heads I am not fixing the car thats going to cost more than the car is worth because there are a few other small issues----I need to know the tell tale signs between heads or heater core----thanks

O.K. Heater core leaking will not cause car to overheat It will if coolant is run low.
If headgaskets are faulty and leaking that will cause car to overheat.
Over heating at low speeds can be caused if cooling fans are not operating. Verify they are operating.