I wouldn't stress about it. A replaced engine means you have another chance at getting an engine that will not break down. I have done two 1600 roundtrip road trips in my Boxster with no problems.
Getting a bad engine in a Boxster is just the luck of the draw. Get you car, break in the engine, and enjoy. Once you pass 25K miles your chances of an engine failure decrease dramatically. But extended warranties are always a good backup.
