I would have to say that by 99 there were basically no common problems to be worried about. Some of the early 3.4's, like 96, had head gasket issues, but they were minor and not a recall item like the older models. Many had faulty gas gauges so that you thought you needed gas before you did, no big deal but probably too late to fix under warrany now. Other than that I would say have it checked by a mechanic like any used car, unless you are comfortable looking it over yourself. If so I would look at all the fluids and make sure they are clean, check the whole underbody for leaks and take it on a long and varied test drive, and then go again to make sure. But a trusted mechanic checking it is the best bet for most. But a 99 is getting to the point where it may be due for some expensive maintenance, it could need brakes soon, hoses, belts, t-belt, shocks or any other number of things. Not saying don't buy it, just know that a 99 will not be problem free like a new car, things wear out.