Hey medic, I had airbags on my 2000 chevy 3500 crewcab and most of the time i would keep the airbags empty and would only inflate them when I was towing or hauling. You can hook them up to a compressor that u install in your truck or you can go the cheap way (like I did) and connect them to a valve (like your tires have) and just fill them up with an air compressor at home or at a gas station. That being said, when they were empty my ride was the same as without them, and when I was towing, the ride was much smoother. As for you and your application, I would assume that you could go the route I did and just keep them filled up all the time, since you likely always are carrying a load.
Yes the airbags lift the back of the van. My truck would lift probably about 2-3 inches in the rear.
For the rear of a truck, they are easy to install. I honestly dont know about a van. for a set up like mine you would mount a bracket to the left and right rear axle and to the corresponding side of the frame. you then put the airbag between the frame bracket and the axle bracket. iirc, you attach the air hose to the top of each bag, have them meet at the middle at a "t" and then shoot another hose to the rear bumper where you mount the air valve. (thats how i think i did it atleast)
i dont know what brands are better than others, but i had firestone bags which lasted 100k miles before i sold the truck
best of luck to ya,