We have a Mazda MPV. It's one of the smallest mini-vans available. The front row is two seats that can be together or apart. The back row is a bench. I have a 3 1/2-year-old, a 22-month-old, and an 8-month old. I have the two older kids in the back, but not right next to each other. One on the left, one on the right, so there is space between them and they can barely reach each other. Every now and then they will reach their hands into the middle and my son will pull on my daughter's arm and make her cry or he'll snatch her toy away from her, but it doesn't happen often and we discipline him when it does. So it isn't much of a problem to have them back there together. The baby is in the front row seat behind the front passenger seat. I have it moved over toward the middle so the two seats are touching. The only problem with this arrangement is that when you put the car seat handle down into the car position it hits the seat in front, so you have to have the front passenger seat scooted up a bit toward the dashboard. If your passenger is tall, they'd have no leg room. (My husband has to drive when the baby is in the car, but I like him to drive anyway, and I'm short, so we don't really have a problem with it. We also removed the handle on the back of the seat to give ourselves another couple of inches. It's complicated, I know, but I figure I drive this thing all the time, I want us all to be comfortable.)
One other thing that has made it so much easier to use this van is we removed the other seat from the front row (they're made to come out for cargo room). We never use it, so we have it stored inside our house. If we ever need it, we can put it back in in just a couple of minutes. Not having the other seat in there gives us so much room to get to the back row and buckle the kids in, and also gives us lots of cargo room, since my double stroller takes up pretty much all the cargo space in the back.
I just looked at KSL.com's classifieds and the MPVs they have listed are priced between $3-8 thousand.
I hope that helps! Good luck!