I think it is better to purchase a Jeep only if you can inspect and drive the rig before making any deal. Typically, buying an "already" lifted Jeep is almost always the wrong thing to do. If you have no experience in building Jeeps, I would recommend strongly against purchasing an "already" lifted Jeep. It's common for four wheelers to attempt to build their own Jeeps with little mechanical skills and knowledge. They either run out of money, screw things up badly, or have patched-up their damaged rigs.
Trail rigs are a money pit; someone else's rig is a deeper money pit! That is why I recommended buying a new Rubicon; it's difficult to ignore the offroad capability of the Rubicon for only about $25,000. You'll probably spend more building a trail rig or repairing an "already" built rig.
Jeep 4WD systems appear confusing. I'll limited the discussion to newer Jeeps.
The basic Jeep Transfer case is the command trac w/ the NV231; this is a good offroad t-case. The new 07 Wrangler and the Liberty have command trac with NV241 t-case; it's the same t-case that comes in the Rubicon without the 4:1 low range, and is a little more HD than the NV231.
Select trac with NV242 is the popular t-case in many Cherokees and G. Cherokees, and is available in the Liberty. It basically has four options: Normal 2-wheel drive, full time all wheel drive, and part time high range and part time low range.
Quadra trac I with NV140 is a simple full time all wheel drive. This is not an offroad 4WD system. Quadra trac II with the NV245 has full time four wheel drive and a typical non-Rubicon low range. The Quadra Drive II with the NV245 is a full time four wheel drive with Jeep's new ELSD available in the Commander and G.Cherokee.
The NV231, NV241 and the NV242 t-cases are your best offroad Jeep t-cases, and, of course, the Rock trac with NV2410R t-case in the Rubicon has the added option of the 4:1 low range.
To recapitulate; buying an e-Jeep is just wrong! If you're not a mechanic, only buy a stock Jeep. Automatic transmissions are dawgs on the highway, but will make you look like a pro when four wheeling on the trail. Quadra trac and Quadra drive transfer cases are not "trail worthy". 35" or larger tires under a Jeep is better done under a Wrangler. Laredo is not a Jeep model, but a trim package. Cherokees and G. Cherokees are light duty rigs, and are not easily modified for serious trail use. Most Jeeps come equipped with marginal axles; 35" or greater tires will require installing stouter axles: installing new axles and taller tires will open a can of worms in regard to other modifications required to safely drive down the highway.
Lockers, that is another post. Basically, a means to acheive true four wheel drive if you install a locker in both front and rear axles. Limited slip differentials are not lockers, and are not effective offroad. Lockers can be automatic (always "on") or selectable types. Detroit, ARB, OX, Eaton, Aussie and Auburn are commonly used brands, some of these manufacturers also sell LSD's. Generally speaking, only install lockers after upgrading your axles. Rubicons have selectable lockers already installed. May Dana 44 Jeep axles have a limited slip Differential, trac loc. Quadra Drive II "transfer cases" depend on new electronic devices to control wheel spin; it isn't exactly proven offroad technology and is only available in the Commander and G. Cherokee.
Any trail rig will be a money pit. Serious four wheeling is an expensive sport. The upgrades and up-keep/repairs are a constant drain, financially. Take a good, long look at this sport before you dive in!
Keep It Simple, buy a stock rig and modify it only after you have some experience offroad and have learned what modifications you'll need to do for the type of four wheeling you'll enjoy doing. Jeeps are "built" specifically for a particular four wheeling application: be it rocky trails, sand dunes, muddy trails, mud holes, swamps and river crossings, tundra, ice-pack . . . you name the venue, most likely there has been a Jeep built to traverse it. There are two types of Jeeps, Wranglers for offroad use, and the other models for highway use (regardless of what Sunder says about his friend that jumps his G. Cherokee over sand dunes <img src="/forums/images/graemlins/lol.gif" alt="" />)!