Rust is the primary thing to lookout for on an XJ. Poorly executed modifications are a big one for me I’m not a brand snob like a lot of people but if someone had done cut folded fenders or trimmed them and it looks like garbage I’ll walk . Obviously check for worn, loose or broken steering and suspension components , check all fluids pay close attention to the coolant if it’s nice and clean green it’s probably been well maintained if It’s Brown id be concerned ... check trans fluid you want it to be nice and red not burned smelling , check engine oil if someone shows up with black oil and low on oil I’d quickly walk away ... open the fill cap while running and check for excess crankcase pressure due to blow by ( smoke out fill cap in excess amounts ), check cold and warm oil pressure , Check the heater core for sure that’s a huge one by making sure the heat works if it doesn’t I’d walk away .... check all power doors and locks ... speakers by testing radio ... AC compressor to cycle when commanded , defrost to work .... check condition of belt and air filter also shows the type of maintenance the owner performed .... check for operation of all lights to ensure ability to pass inspection ... condition of exhaust ... I can keep going but there’s a bit of an art to a good used vehicle inspection .