1998 Buick Regal anti freeze fumes in my car

My car just recently started messing up here and there. My car is misfireing. I found out P0205 is the computer code for it. Could changein the plugs and cords fix it. (Or is it easy putting new injectors in, is it exsensive. Or put new "cords?" On it. Kuz they did look wore out. And is that easy to put on and is it exspensive?)

and I think my thermosat is going out. I cant find any coolant leaks when the car sits. Guess it comes out when im driveing. (Low coolant comes on here and there) but it sounds weird when it runs.

the inside of my car smelt like coolant. It was pretty strong.

and when I start my car and put in reverse I here a kinda hittin sound. But its a split second sound. I think its my brakes. Cause when it does that my brakes get real stiff. And at night when I put it in reverse it makes my headlights kinda dim real quick?

CAN ANYBODY HELP ME!