While there are incidents of failure, consider how many folks who have no issues DON'T go out of there way to post their non events. Squeaky wheels do get the oil
If you are worried about engine longevity, you might consider an aftermarket warranty. Then again, if nothing goes wrong, you are out big $$$
Some
don't consider it a move up from a Cayman to a 997
. . . but since you are a current Cayman owner, how worried are you about its motor failing? If the cayman motor doesn't worry you, the 997S motor shouldn't either.
If you are REALLY worried, try on a TT or GT3 instead.