New SC2 Owner
 

I just purchased a 2002 Saturn from a friend who was moving. My husband would say it was an act of charity which I am inclined to agree with (we paid her $2000) As soon as I drove the car I loved it and now I really want to fix it up and make it my baby.
Right away It needed new brakes-everything and bearings in the back. I went to the shop for that and all my mechanic friends are ready to kill me. ($800). I am still hearing a strange whirring sound sometimes when I brake. It kind of sounds like an antique car horn. I am inclined to believe that it has something to do with the anti lock brakes, but wouldn't have that been fixed in the brake shop?
Right now the check engine light is on and we have had the codes read and it seems that most of the sensors are wack. (coolant, gas recirculation, Camshaft, and electric exhaust recirculation)
My question is how hard is it to change these sensors. I looked up the htcs and it looks easy enough, but how easy is it actually. I am not a car gal but would love to learn more. I was thinking of changing the spark plugs too, what kind to you recommend? I looked on the new owners post, but the links to youtube were no longer there.
The car has 218, 000 miles on it but they were mostly highway miles. I would love to see how far it can go but I don't want it to be a money pit.
Any advice on the order of repairs or up keep would be appreciated.

Use OEM parts for any sensors you replace .. USE OEM type spark plugs as well.