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Generally, after about 35-45k miles if IMS was going to happen it would have already happened. But it can still occur, and it has. IMS is not that common, but you could look into a $3k warranty, or just put some money in a high interest bank account and let it buildup. Personally, I think the warranty is the way to go because it will cover everything and not just an engine failure. (for info on warranties do a search)
Also, if the local shop has experience with Porsche's I don't see why you would have a problem with them.