There are only 6, how many is just a few? It could be anything from a bad head gasket ($600) to broken pistons/bent valves/cracked block (2-5k). It's not the end of the world and not all that expensive to rebuild relatively speaking but question is why? Is he interested because it's a pristine example of the exact color etc that he's always wanted, or (more likely) because he's poor and it's cheap? If he's interested because it's cheap, let him rest assured that there will be more things wrong. This is not the car to look for to try and score for cheap.
Here's a valve from my engine with "just a few cylinders with low compression"