The consensus has been to stay away from the 99 model year, as they had problems with the launch of the WJ. Both the I6 and V8's that are offered will last a long time if taken care of problem. You should try and find a V8, as there isn't much of a gas mileage difference, both engines last for the same amount of time, and the V8 has a tremendous amount of power compared to the I6. Either engine should last 200K+ if properly maintained. Just be careful like others have said about the all wheel drive transfer case viscious coupling (NP249). Also, drive it around a little before making a purchase and see how well the transmission shifts. Again, they are fairly reliable vehicles, but you never know how someone has taken care of their vehicle. Good Luck.
1996 Grand Cherokee Ltd.
3" Skyjacker lift
2004 Jeep Liberty Renegade (stock for awhile)