Leaks and rust.
Fold down the back seats, lift the carpet and look at the boot floor. I'd bet that the insulation will be soaking, and the floor very rusty.
The good thing for the buyer is that this looks really scary, but is very easy to fix. You might get away with wire brushing then coating with Hammerite, but even if it's completely rusted away, a replacement floor panel costs about 60 quid. The water is almost certainly coming in thru the sunroof or the gutter join.
But it looks scary, so is a very good negotiating point when haggling over price!
Also, check when the timing belt was last done, and whether the timing mod kit has been fitted. A blob of yellow paint on the timing cover usually, but not always, means the modified kit has been fitted.
Bruce.