http://www.pelicanparts.com/techarticles/944_buyers_guide/944_buyers_guide.htm
Written for the 944, but it applies to the 924S as well.
The 924S is significantly more reliable than the original 924 - it is essentially a 944 engine in a 924 body.
Before you become totally frightened, the repair prices listed are if you have a professional Porsche mechanic work on the car.
Many repairs can be done in the driveway for significantly less cost (some you will need a mechanic - repairs that require special tools etc). you will need a good set of metric wrenches and sockets. plus other various tools as things come up.
Get the best one you can afford, a "cherry" one for $4500 will save you a lot of pain in the wallet over a $1500.
In any case, have a prospective purchase inspected by a qualified Porsche mechanic before you buy. Hidden maintenance costs can double what you have invested in the car very quickly.
Welcome to the group
AFJuvat