Start off with where you live as diferent areas have different problems (as in rust).
Stay away for the 90's and down as they will have a 4.2 which runs a carb. You will like fuel injection better
Most peoeple are not happy with the 4 cylinder engine, that is your call.
All stock YJ's have a D30 front axle and this can be beefed up to work for a long time depending on what you want to do. They also all have a D35 rear axle (this is a very weak link)
It all depends on what you want to do with it.
Mine is still Nearly Stock
But you may want to modify yours some.