We've had our 1999 amigo since the kids were 10. They NEVER had any issues climbing into or out of the back seat (including using the rear moon roof when that was removed!). As far as what you can carry, we could load up the Amigo with all of our gear for a three day trip to Moab, including a cooler, with two kids in the back, a dog, my wife and me. That was for hotel camping - so our gear consisted of whatever clothes, etc. we would need for the three day trip, plus my normal recovery gear and tools. It was packed pretty tight in the back behind the rear seat, but the kids had plenty of room for the 6 hour trip there and back.

As far as towing, we used ours to pull our popup trailer all over Colorado. It weighed in at under 2,000 pounds fully loaded, and the amigo did just fine (pretty slow on some of the big hills here in Colorado though). I personally wouldn't recommend much more weight if you are going to be hitting any big hills. On our last trip to Crawloween, we had the amigo loaded with gear and kids (both 17 years old now), the dog and me and the wife with a Samurai behind us on a tow dolly. That load really slowed us down on the steep hills (3-4 of them between Denver and the Eisenhower Tunnel, plus Vail Pass).