few things to check(having sailed Dart 18 for 10 years)

1, I would suggest changing the rigging anyway if you decide to buy. If it's been around a while I would just bin it. The rigging is not great and I would change mine every 18 months or so.
2, How old it the boat?
3, Is this 3 inches when the boat is tippeed back or when flat. 3 inches when tipped back is not much. 3 inches when flat is a lot.
4, Tip the boat on it's side (on grass or sand, not stones as the sides of the hulls are very thin and will get damaged.
5, What state are the skegs in? Is there glass showing? If glass is showing is it soggy, if so it's a fairly big job to cut it all out, fare, add mat, fill and re-gel (but not difficult, just time needed)
6, What state are the rudders in? get them out and see if the move well or are they in a bad way? Are they full of water?
7, Are there any stress cracks along the deck seams (one place they may leak is around the toe loops on the hulls. Again not a massive job to fix, but worth taking into account
8, Does the boat have all the blocks present, what state are they in?