Found some used Kooks catless headers for a fair price, have 40k on them. Aside from cracking what else should I look for? They were on his 2016 but shouldn't have any trouble fitting my 2018 correct?
Kooks makes great headers and uses quality materials, so I wouldn't stress it too much. As you said, look for cracking and any other odd signs of wear. Also, just like grimace said, make sure you replace the hardware.
Tip: Before installing the headers, I would get them coated or wrapped in exhaust wrap. Definitely not necessary, but I wish I would have wrapped mine to keep the engine bay cooler.