It really depends. Jeeps are very reliable vehicles, but things do go wrong. I had a 1997 TJ for 8 years and never needed any warranty work aside from a bad steering box that needed replacement right when I got it. Apart from regular maintenance, I never had anything go wrong until I was well over 60k miles, and those were minor things like a leaky pinion oil seal.
My 2005 Rubi had its throwout bearing go out right about 2800 miles. I really don't anticipate any major problems in the future though. The 4.0L I6 is solid. I may break stuff on the trail, but that's just how it goes.
My girlfriend drives a 2001 Volvo S40, and she spends far
more than I do on regular maintenance and fixing things that go wrong. But that's not just because I have an '05. That's including the 97 I had up until 6 months ago. Your mileage may vary, of course, but I think you have less to worry about with a Jeep than some other kind of vehicle. I'm on my third
As for used stuff, ditto on looking to see if it's been beaten up on the trail. Check the oil pan, differentials, t-case skid, etc for dents and signs of rock-bashing, and if it looks excessive, there could be problems lurking. If it's been a street Jeep only and it feels good to you when you drive it, you're not likely to have problems with it.
Good luck to you!