Look for the same issues that you would see in most any vehicle when shopping them: Leaks (the 4.0L six is prone to rear main, valve cover - which can be misdiagnosed as RM seal leak, etc) shimmies & shakes, tracking, look underneath at the frame areas for covered up damage from accidents, smog equipment intact and recent good smog check, function of transfer case - listen for chain noise (slapping sound) diff gear whine, etc.
XJ's are pretty robust and they can last a long time. Let us know what you find and post up links, pics and we can help you spend your money.
