Most important thing to look for is signs of diesel in the sump oil.
Look for elevated oil level on dipstick and smell the sump oil. If it smells of diesel be wary.
If its there then knock £3k off the asking price or walk away.
At the very least it would need a new cylinder head, and possibly a turbo, and/or bottom end rebuild.
Have a look at the cylinder head. If it has an external fuel return pipe running from the front of the head along the top of the inlet manifold to the fuel regulator at the back of the head then it has the later strengthened head which does not suffer from the fuel leakage issues. Engine numbers beginning 15P and above should be OK.
Other than that then normal rules apply as to what to look for. Rust, oil leaks, etc.