Let’s start with the cars in the first group, all of which are at least 20 years old. The possibility of the IMS problem occurring appears to increase with mileage and vehicle age; if the Porsche you are considering has average mileage (6000-8000 per year), you should have the vehicle inspected for common issues that would almost certainly have occurred at this point. That said, it is still good to keep the problem in mind when buying and/or maintaining a Porsche. Make sure any car you are considering has full-service records, and if an IMS failure has occurred, that it was properly resolved. More frequent oil changes than suggested by the factory, are a further precaution that can protect against failure.
As mentioned, there is greater risk with the 2001-05 cars. However, considering these Porsche are at least fifteen years old, the chance of future failure is relatively low. If you are considering the purchase of a Porsche that has more than 40000 miles, and a review of the maintenance records shows it has been meticulously maintained, the risk is very low.
The risk for the third group, 2006-08, is similar to the first group in terms of the likelihood of failure except these cars are newer and haven’t had as many miles on the road. Still, all of these cars are at least 12 years old and should have logged enough miles to flush out most of the problems. You may also be able to purchase an aftermarket warranty, which could be a wise decision if you are troubled by this potential problem.