Purchase of 1997 LeSabre

I am looking at purchasing a 1997 Buick LeSabre Custom 3.8 V6 (70,000mi) from a local dealer. I looked it over yesterday but couldn't test drive it
because the dealer had only just got it in and they had to do their "safety check". Today I called the sales person and he told me that the car had failed the check because they found that the intake manifold gasket was leaking, but he told me that they would get the gasket replaced in the next couple of days and that he would call me to schedule a test drive as soon as the work was complete.
My question is why would an intake manifold gasket fail after just 70,000 miles? Is this a known common problem with this engine, or could there be a more serious underlying cause? What else should I look for?
Several people (with more auto knowlege than I) have told me that "I can't go wrong" with a LeSabre, and that the engine is one of the most reliable in the industry, that's why I am looking to purchase one. A problem like this does have me concerned though.
Any input would be appreciated.
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