The ZJ Grand Cherokees are a bit quirky at times, but they're not unreliable. The only real issue a V8 one is guaranteed to have is a plenum gasket failure, which isn't major to fix. Suspension parts and such do wear out, but not in any unusually short period of time (I did ball joints and tie rod ends around 130k, my control arm bushings are still fine at 142k). Other than that, if it's well taken care of, they hold up pretty well.
Parts for these things are fairly cheap and pretty much everywhere.
The transmissions aren't nearly as bad as people say they are, unless they're abused. Put a big trans cooler on it and keep the fluid fresh and it'll be fine. My trans is still original with 142k. I added a huge cooler at 102k, shortly after buying the Jeep, and put a shift kit in at 120k (about 1k after the engine power upgrades), and it still shifts better than new and has no problem barking the tires into 2nd at WOT.
Also, if you find one of the 98 5.9s, the 5k lb tow rating (compared to 6500 for the 5.2) is only because of the electric fan. If you have your head on straight, it'll be fine at 6500 (I've done it for 370 miles in 75* weather), and if you're worried, it's easy to retrofit the mechanical clutch fan (and there's a TSB somewhere that up-rates the tow rating to 6500 with a clutch fan).
Also, my general opinion is the vehicles in the 100 - 150k range are those to be most scared of. Under 100k, it's still young. Over 150k, if it's still in good shape after a thorough examination, it was likely well taken care of.
