Things to check, depending on milage: the higher the milage, the more important

CV boots/joints cracked boots and grease leaking is bad, clicking joints are worse

Oil leaks - valve cover, dipstick, anything that could be related to crankcase vent valve

Check all hard parts for leaks - transaxles, transmission, engine, transfercase. That means you may have to look beyond the stupid skid plate, which is a pain. It will conceal many leaks.

Proper operation of the climate control (points to final stage resistor, compressor issues)

Whoever owns it will probably have put new tires on it to sell it, but if it has tires with wear on them, check the wear for eveness. If it is not even, the front end suspension needs work, or at least an alignment.

Power steering high pressure hose - it is rubber with metal links; this design tends to leak

See if the following have been replaced:
<these are potential problems I and others have encountered>

crankcase vent valve
waterpump
thermostat/housing
cv joints/boots
FSR
fluids - somewhat of a personal opinion at this point, but it would be good to at least know if the factory fill was still in it or not

See what the maintainance history is on the following:
<no one likes taking a car they just bought in for hundreds of dollars worth of maintainance>

oil
brakes/fluid
AC


I am sure I am forgetting some things. But those are the ones I would want to know about for certain if I was looking at an X5 in particular.