Hi Chris,

"cracks on the tubes" are common on a Dart 18 and nothing to worry about. I guess the only way to check the hulls is to sail it before buying. After one hour or so, you should not have more than 1 or 2 pints per hull. If the wires and shackels have rost, kinks, or other strange appearance replace it. Halyards can be really old, since you use them on beach and hence no big pb, if they fail (on the water the sails are hooked).
Check the sail battens, they break after some time and penetrate the sail. If you see lots of fine cracks at the part close to the luff, it is time to replace them. Check their price on www.lasersailing.com, you may be surprised.

You will love the Dart 18, especially if it blows.

Cheers,

Klaus