I also own an xj...
as has been said, the owner of it likely does not know what he's talking about when he says V6, unless he's put a new motor in which is unlikely - it's the same bomb proof I6 4.0
I would be interested to know what kind of control arm setup he has on there, stock short control arms would make for a harsh ride, a drop kit would be better but is kind of bulky, and long arms would be ideal.
If or when you drive it listen for squeaking in that area, if he put a poor quality lift on it the bushings might be bad. If you feel like the steering is very sluggish and the jeep wanders on the road, I would recommend staying away from it. Steering issues and worn out bushings can be dangerous, not to mention a likely cause of death wobble (probably one of the more frightening things you can experience driving a car)
Obviously death wobble can be fixed but it can take a LOT of labor hours trying to determine the cause.
Other than that, it's a Jeep. It probably leaks I know mine does. Make sure you check if and where it leaks from.
Check the tires for any sidewall damage from offroading... 33" and 35" tires are not cheap to replace if they pop, and also that much rubber loosing it's form going any highway speed can be an instant rollover.
If you do get the xj, check out naxja.org for a wealth of information but be prepared; you will get hated on for being dumb there too.
Looks like a sweet rig though!