They have the following problems until 2003.
1 Trannys are very weak and have sealed for life units, meaning you have to pay for a new transmission.
2 There's a timing chain guide thingy that's made of plastic, it gets brittle and breaks. When that happens you're engines toast. You can have it replaced with a metal one pretty reasonably.
3 Coils, you'll most likely need new ones if they've never been changed.

Thoses are the major problems, but there's a few more including the power window motors. If you're going to buy this car, I would recommend finding a mechanic and purchasing a warranty from them that they'll accept. If you don't have $5K for "just in case" money and can't find a warranty, I'd personally pass on any pre-'03 S-Type