Take a large flat head screwdriver and poke around the frame where the gas filler hose passes thru, poke around the foot wells - especially the driver side foot well, poke the rockers at the rear door just in front of the rear tire. These are all very common rust areas.
When you inspect the engine and under it, a small oil leak at the back of the engine is probably the valve cover gasket leaking down (engine tilts to the back) and is most often NOT the rear main seal. Easy fix.
It would also be wise to get the front axle up on jackstands and verify the condition of the tie rod ends, the ball joints, and the unit bearings. Easy to do if you have the time and tools, expensive to have done by a mechanic.
Put it in 4WD on gravel or dirt or grass and drive it turning the wheel to full lock in both directions. This will confirm that it can shift in and out of 4WD and also give you an indication if the axle shaft Ujoints are shot or not. As above, not hard or expensive if you can do it yourself. 20-30 bucks in parts per side, but multiple hours if a shop bills it by the book.
Take the rad cap off and look at the condition of the coolant. It should be bright green, not rusty or oily looking.
Remove the Oil Cap and look inside for excessive sludge, signs of coolant contamination (white sluge - a bit can be from winter short drives and moisture. A significant amount of white sludge is a bad sign that the head gasket is gone.
Take a look at the rear bumper caps where they cover the lower quarter panel - this is a good place for rust to start as well. Rust around the front windshield frame is not necessarily common, but happens.
Oil Pressure should be 50+ under load, and can settle to low teens at idle when warm. Nothing to worry about for a 4.0L engine.
Look at the rear axle - if the bottom of the axle housing is a flat lip as opposed to roundish it will have a good 8.25 rear axle. Inspect the front and rear pinion seals as well when you're under there. They are common leaks, and once again are cheap parts, but need to be done correctly or it gets $$$.
I've been a bad boy to my '00 XJ and pounded on it wheeling, but always maintain the cooling, engine, tranny, and axles. I'd drive it across the country if it wasn't such a pig on fuel and not as fun to drive on the highway as it was when it was stock many years ago. I trust it to be reliable explicitly.
Good Luck with the Purchase.
'00 XJ - Somewhat modified
'01 Venture - high mileage, paid for, shut it