I6 (straight 6) usually refers to the 4.0 L That engine is used on many Jeeps, including the XJ's. Good engine. If you are going to buy a high mileage Jeep, then just remember that you will have to put some money into it to replace ceartain things, just like you would any other used car.
Make sure the Jeep you get is mechanically sound, and you will be okay. Check out the engine and the transmission (transmission), and make sure it shifts good. Check for rust spots, and do a Vin # check. Just the usual stuff. Hope this helps.-al