The best kit (whatever it may be) will new coils, new lower control arms, long MJ-specific AALs for the rear in addition to the brake lines, sway bar fix and trac bar fix (those can vary as to how they acomplish them). Plenty of kits won't have all of those. The only thing that typically won't be included is shocks and that's because the manufacturers recognize that many people have differing preferences. XJ kits will generally come with t-case lowering blocks, but MJs won't need that.