18s definitely fit. They were an option on the GT and came with 235/50-18 tires. It's more a matter of the width and offset if different from factory on the S197 whether they will fit or not. It's much simpler and less complicated to get a factory wheel and tire size that you know will fit with no problems but you can typically go slightly taller and/or wider than stock to a degree.
If the height of the tire is different then what came on your car from the factory then you will need a flash or tune from the dealer, Forscan software, or an aftermarket tuner to correct the speedometer. However, if it's only within 1/4" difference you could leave it as it will be pretty much unnoticeable.
I believe your car can comfortably wear 255/45-18 all around but anything wider than that and you may get rubbing and clearance issues at least in the front.
huberoy123's link is good to find a suitable fitting wheel and tires as well. You could also scour craigslist, ebay, and FB marketplace to find used take offs nice and cheap. Maybe you'll only need tires in such a scenario. Wheels are one of the most popular changes made when buying a Mustang so used 18s should be easy to find.
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