The hesitation could come from a couple of things. One would be a simple fix which is a clogged fuel filter. The other issue could be bad coils or a combination of both. From what I've seen and experienced it could be bad coils. Is the check engine light on? Have them run a diagnostic on the car to see if there are any stored codes. This will let you know what the car will need in repairs.
As for the ball joints, that is a fairly expensive repair since the ball joint is part of the knuckle. If you are dealing with a Lincoln dealership you may want to have them include the repair the hesitation and ball joints as part of the sell. GET IT IN WRITING IF YOU DO AGREE ON IT!!! The ball joint repair will be in the $700 range so it could get expensive.
Also, don't jump on the first LS you find. There are many, many good LSes out there for you buy. I would hate for you to get one that had some problems and then regret buying later. They are really great cars and spending $1000 or more on repairs out of the gate may leave a bad taste in your mouth in regards to the Lincoln LS.
