I would recommend reading all the information in the lift and tire sections of this forum
Especially the "Stickies"
Almost all of your general enquiries will be answered in those pages of information.
Then when you sort of narrow down what you want, read all the other threads.
But, For what it is worth, do it right the first time and really look into replacing your factory suspension.
Especially if you have over thirty thousand (30,000.00) miles on your factory suspension. By then the factory coil springs have sagged, bagged out/worn out. The factory front struts and rear suspension dampeners/shock absorbers are also pretty much at the end of their service life.
Old Man Emu or (OME) coils are what you really should consider.
Weather you opt for OME suspension dampeners/shock absorbers and front struts or the ever popular Rancho counter parts, that is up to you.
Then of course there if the "Spacer lift (s)" which fellow KJ owners also use!
Ask them directly as to what they think of them, how they perform, or how they don't perform. (It all depends on what you are using your Jeep Liberty for!)
I, have waited for two years and going on six months to buy my after market suspension componentry. At first I was going to buy a spacer lift kit (DAYSTAR) but continued to hold off until I could buy the real McCoy!
A full Coil spring/strut/shock lift kit. (I have been waiting for just over a month for my goodies to arrive to my home here in Canada!, But, then again, what is another six or eight weeks? After waiting for two and a half years to get what I really want?)
Take your time and research every available bit of kit you can buy for your KJ (Liberty).
Everybody here will let you know in their own special way weather or not you are making a good decision!
I hope this blerb from Ryan has helped you a little!
(Cheers)
Ry'