It probably could use a tune-up including sensors but the shotgun approach isn't cheap. That's why the code dump could be useful. Does your check engine light show when you first turn on the ignition? They can burn out.
Also, codes are thrown differently between vehicles. Many times they may register without causing the light. I haven't worked on a Pontiac since before OBD so couldn't tell you. The store requiring the light first tells me they're trying to discourage diagnostics that don't sell parts. Try another, another shift of workers, or stoop low and say it lights sometimes and your mileage is really bad. There's always the "pull a plug wire" method so it sounds like death on wheels.
Your best source of info will include people familiar with the vehicle. That usually means an enthusiast forum of some kind. As always there's good and terrible advice, but there's usually a core of people to guide you. FAQs for things like "declining mileage" and "basic DIY."
I can't point you to any but google would be where I'd start. Google something like ("pontiac sunfire" forum enthusiast -dealer -sales) without the parentheses. Or just go pour coffee and sift through "sunfire mileage problem"...
Oh, and beware haynes & chilton. Better than nothing, barely. Look for factory manuals even if it means ebay. The best money you'll ever spend. (Assuming you're keeping the vehicle for awhile.) Even if they do cost a bit you can turn a used set back around on ebay for what you paid.