The KV6 is very susceptible to death by overheating, which is a problem as its cooling system is extremely fragile. Many components are made of brittle plastic.
A compression test is required as part of a prepurchase inspection. If the seller mentions anything about losing coolant, then run away.
Intake camshafts are brittle and have a tendency to sheer off near the pulleys – this requires replacement of the camshaft, and can also potentially cause damage to the valves and pistons. This can be caused by improper timing belt changes, too.
The engine uses 3 timing belts, which means that a timing belt service is about $1700-2000. Due to the specialized tools required, this should not be conducted by an amateur mechanic. Before buying a car with this engine, ensure that either a FULL (not partial) timing belt service has been done, including tensioners and guides, or budget the money to do so.
The intake manifold flap motors tend to fail. These are cheap and easy to replace. What is not cheap and easy to replace are the flaps themselves, which have a tendency to break. The only solution is an entirely new upper intake manifold, which is about $1100.