After having spent almost 4 years in the car business, albeit with a Japanese manufacturer, I can tell you with complete certainty to walk away if they wont let you take the car to an independent mechanic for a pre-purchase inspection. Where there's smoke, there's fire. No dealer in their right mind is going to turn down the chance to sell you the car by denying a PPI unless there is something they don't want you to find.

Just my .02

Jonathan