I have the 4.0L I6 in my '96 Cherokee - great motor, as just about anyone who has ever had one will tell you.
My wife has had her '04 Grand (4.7 v8 / 5spd auto / full time 4wd) for 2 years now and she loves it. Only issue we have ever had with it was some odd electrical issues with the driver's door switches - turned out to be the ground wire passing from the body to the door. It had been completely separated, but it was simple for me to fix in about 30-40 mins taking the door apart my first time.
My opinion would be that she couldn't really go wrong with any 02-04 I6 or v8. If you are gonna get stuck helping her out with it, go here:
www.wjjeeps.com Tons of basic info about the 99-04 Grand Cherokees.
- Justin