Re: Service advice
I, personally, would not replace the spark plugs at 18k miles. Keep in mind that with average driving, a 5 year old car would have 50-60k on it. That's a lot more spark plug firings an wear than you incurred. If they were worn or faulty, I would expect CELs errors. That's what my Audi does when it misfires.
Otherwise air, gas, and cabin filters is all that i would change. I believe fluids are good for 100k miles, but you should check the maintenance manual to be sure.