Some basic tips for the original poster:
If you're buying one that's modified, be careful because some people don't know what they're doing or cut corners. I personally would not buy one that was heavily modified. new springs, shocks, bolt on additions are usually ok. but if you see one with a 3 inch body lift, I'd steer clear. Make sure it looks proportional. If the tires are way too big or too small for the amount of lift, you might question it. I'd also ask if any gearing was done if the tires are 35" or bigger. Obvious stuff applies too, so check for rust, paint overspray, torn seats, wires in odd places, and basic fitment of things like doors, hood, dash panels, etc. Have the guy or girl selling it to let you take it for a drive, and maybe have them take it to some loose gravel and check all the parts of the 4x4 system. check the a/c and heater. finally check out autotrader, edmunds, and kbb for value. you don't want to pay 10K for a jeep that is probably only worth 7K. Good luck!
2003 Rubicon (shale green/khaki), 33x12.5x15 BFG Mud KM2s on Pro Comp 8179s (Firestone Winterforce on stock Moabs for winter), Bilstein 5100s, 3" AEV (nth) Progessive coils, OME steering stabilizer, 1" M.O.R.E. body lift, JKS adj. track bar in the front, 1" JKS budget motor mount lift, RC quick discos, OR-Fab rear rock crawler bumper with 2" receiver, Rokmen 3/16" steering box skid plate, Rokmen rear frame tie-ins, Teraflex 3/16" heavy-duty oil pan skid plate, Alpine CDE-124SXM CD/sat radio MP3 player, Bestop sliding windows, Bestop Safari Bikini top (green military camo), Mopar "windjammer" rear window, Mopar "duster" tonneau cover with tailgate bar, Rockhard Full roll cage with full padding, KC slimlites long range/off road lights on KC windshield mounts, Tuffy security glove box, Pilot LED tail lights.