You will really enjoy the 172. It's a nice balance of capacity and compactness. If i wan't old and stiff, we wouldn't have sold ours. According to Mr. Google, the Sienna has a towing capacity of 3,500, do you'd be very close to or over the limit, especially when you add passengers and your stuff. You may also find that when climbing in the mountains, the Sienna will be pretty stressed. If I were you, I'd check with an honest mechanic to see what you can add on to help, but keep in mind that for practical purposes, it will not increase the towing capacity, just keep you from destroying your engine and/or transmission. You will also need to know the combined gross vehicle weight for the Sienna. That's your real limit. You may keep the trailer nice and light, but with passengers and such, you may find you're over that limit.
Good luck in your search.