Two car seats fit fine in the KJ. If you put three in there, you'll probably have to get a specific model of seat so they'll fit. The JK has a little more room, but not enough to make that the deciding factor. Get the JK Unlimited if you'll be happy with just that (no mods). Get a KJ if you want to personalize it. Get a used one if you want to personalize it more.
Here's a high-level recap of the KJs life-span.
Front ends: They changed the KJ front-end in 05, so get an 04 if you like that older shape. The 05 and 06 Renegades have a flat hood that looks pretty cool. That's about all that's changed (major) on the front end of the trucks through the life of the model.
Exteriors: Sports and Renegades have black plastic fender flares and Limiteds have body-color fender flares. Renegades have "rivet" points in the flares. There are some off-shoot models that combine body-color bits with black bits, but those are the basic three models of the KJ. The Sport and Limited have the same roof rack rails, but the Sport's are black and the Limited's are chrome. The Renegade has thicker roof rack bars, but they are rated to hold less weight (go figure). The Renegade will most likely have a 4 light bar on the roof. A basket, or "Tupperware" thingy was available for the Renegade. Not sure of the specs or usability of this item...I see a lot of them for sale. The Renegade is not available in 07.
Engines: There have been three engines in the KJ; 2.4l 4 cyl., 3.7l V6, and 2.8l 4-cyl turbo diesel (CRD = Common Rail Diesel). There was also a 2.5l CRD available in Europe and Australia, but was never available here. The 2.4l 4 cyl was the base motor from 02 through 05. In 06 they dropped this engine altogether due to low sales volume. The 3.7l was an available option (included with some trim packages) from 02 through 05. It is now the base motor for the KJ. The CRD was only put into the 05 and 06 Sport and Limited (in America). It is not available in the 07 models.
Interiors: There hasn't been much change in the interiors over the span of the KJ. In 05 (or maybe 04) an OS panic bar was added on the dashboard in front of the passenger. My wife loves that thing. The power window controls apparently switched a bit in 04 or 05. The window controls are on the center console in a 2+2 layout. On earlier models, the front window switches were the furthest back, and the back window switches were the furthest forward. In 05 (or maybe 04) the swapped these so the front window switches are the furthest forward and the rear window switches are the furthest back. The seats in the 05 and up also fold "flat." I put that in quotes because they are still not completely flat, but apparently they are MUCH flatter than the 02-04 models.
Mech: The early 02s sat 2" higher. If you're putting a lift on, I wouldn't consider this when buying...you're going to replace all the parts that made it 2" higher anyway. The 02-03 had drum brakes in the rear and discs in the front. I believe they put discs all around in 04. I know that the 05s have 4 wheel discs. The 06s and 07s have ESP traction control. Manual transmissions are hard to come by. They were not available on CRDs (in America). If I remember correctly, in 05 you could get a 6-speed manual mated to the 3.7l V6.
I think that's the major points. There have been some transmission changes over the years, but I'll let someone who has more knowledge touch on those.
Good luck in your search.