How do you fix something called a nox censor for oil in your car?

our check engine light is on pretty much all the time. we took it to auto zone and had them test it. they said it was something called a nox censor. it costs about 90.00 to buy. today our car started smelling oil and then it stalled on a major highway. we got it safely to a parking lot and we sat there for over 2 hours before we got it running again. we put water in it. and tried to cool it off. we bought it on easter sunday. of 09' and we have had nothing but problems with the thermostat and other things since we bought it. it is a 99' subaru outback legacy. sometimes we have white smoke come out of the tail pipe. which tells us something is also wrong with the head gasket. WE NEED HELP FIXING THIS CAR. HOW MUCH WILL ALL OF THE REPAIRS COST. I AM DISABLED AND MY HUSBAND WAS LAID OFF FROM WORK. SO A UNEMPLOYMENT CHECK EVERY 2 WEEKS IS NOT A LOT. WE ARE BARELY GETTING BY.