The Ford Windstars with 3.8 have headgasket problems more common than intake leaks on our motors. I had a few neighbors with them and they were always in the shop for it. My buddy with an indpendent shop told every customer that came in with that problem that he would fix it once and told them to sell it.
Dodge is known for transmission problems. When I was helping a friend look at minivans a few years back it seemed they were at a discount (we assumed because of the known transmission problems). He was able to get a much better deal on them and decided to take the chance and worse case would have a transmission rebuild at some point and the cost would be the same as buying the Ford or GM equivalent. So far no transmission problems and he is happy with it.
I would assume owning a Grand Prix you are generally familiar with the drivetrain issues (or lack of) you will be looking at with the GM minivans.
If you consider foreign. RUN from Honda. I have worked with several people who had them. Transmission crap out at 30k miles like clockwork. Honda only covers the first one if your VIN falls within their bulletin. I assume in your price range there is 0 chance of warranty. I know a guy who used to work for Honda. He said it is an internal design issue that cannot be remedied without a new case that has a different design. I never got into the details, but trust him. Considering he sold his Honda Odyssey (or however you spell it) and swears he will never own another Honda.
I think in your price range, I would seriously consider the condition as important as the specific model. A well maintained Chevy Venture might be better than a poorly maintained Caravan. I personally would be leary of the Windstar since the headgasket issues seem to be recurring, but you may feel differently.
