Almost any car can be reliable if it has been well maintained, and especially if low mileage. Likewise any car if abused and neglected will eventually break down. The S-Type is no better or worse than other modern cars on this score. Parts can be a tad more expensive that some, but you don't normally buy a Jag for its economy, but for its class.
If you like the S-Type (and I do, very much) then get one, but get a good one. You'll love it.
"You remember the quality long after you've forgotten the price"