The best way to determine what needs to be changed is to visually inspect it. It is extremely easy to pull a spark plug, once you do, it will be pretty obvious if it has been fouled or not, if they are platinum tipped plugs, they should be fine. Also check you plug wires to see if they are cracked or dried out, more than likely they will be fine. Since the car is running well and you haven't had a drop in mpgs the plugs and wires are probably fine. The transmission fluid should be red or pinkish, if its dark or brownish its time for it to be changed. If you have never changed the fuel filter that would be something to definetly do. Cooling system should be flushed (especially in a northstar) if you have never done it, or its been two years. A very overlooked fluid is the brake fluid, if its original I would have the brakes bled. You can buy test strips at any auto parts store to see how bad it actually is.