A PPI can give you a basic idea of the condition of a car you might want to buy but it will never give you everything. No one can see through paint, or inside an engine, so it's only useful up to a point. If the car is a real turd that should be fairly easy to spot but what if it's a rough diamond? Not everyone car spot a really dirty car that has good bones.
The only way to really know a car is to have worked on it. Service records help but if there are none, or the car doesn't run, what then? Your Euro car garage will begin to fail quickly when it comes to the fine details that a specialist would spot. And even then, you still won't find everything wrong. Just factor in an extra 25% for unknown costs and you might be marginally close.