First time I sailed my Nacra I took in about 2 cups of water over 4 hours - and I freaked out! But helpful posters on catsailor and the folks at SidSports told me not to worry; that was probably a slow leak in the drain gasket.

The Prindles don't have daggerboards so well leaks aren't a problem. And if the fiberglass appears strong, it could be just a gasket leak. That can be fixed pretty easily; and depending on where you live $900 might be perfectly good for the boat, especially if the sails and tramp are in good condition. Here in Utah cats are sufficiently rare that $1500 is a decent price for a mid-80's model in good condition.

But our fellow posters (with more experience than I) are right. At the very least, the owner should get that leak fixed before selling. Unless you can be guaranteed the hulls will last, keep looking.