It's a bit difficult to provide a quantitative answer on the info provided Tim.
It might be a sensor, it might be the ABS unit... or it might be totally unrelated, as some faults are on the S-Type.
Need more detail on the car I think. Price, age spec etc.
My initial reaction would be, if the seller won't fix it, walk away, there'll be another around the next corner, as it were. But, if it might otherwise be a bargain...
Welcome to JoC btw :)