We have the Peugeot 406 (2 litre TD) with the child seats in the back, it's a lovely car to drive in (except for an inexplicable unlocatable draught in your face in the front passenger seat) but when you have the 2 seats up you can get very little else in. (Plus it makes both our DSs travel sick, I think it's the smooth but soggy French suspension, DS1 always was but DS2 wasn't, and isn't in my car, so it's a point to consider!)
We wanted the Audi A6 - very similar in total space - but it was being restyled at the time and there weren't any (it's a company car so there was a deadline). You can get them with child seats in the boot but I don't know how easy it would be to find one second hand.
Both of them have a huge capacity for camping kit!
We used to have a Toyota Space Cruiser, which was the one before the Previa, it was narrower but the same basic design, much larger luggage space than all the Galaxies etc and you didn't have to remove the rear seats, you could fold them away to the sides - if you just fold one away you end up with a huge L-shaped space. (And it had belts for 8 although the 2 centre ones were lap belts.)
There is a 7-seater Toyota Avensis Verso now, kind of a replacement for the (6-seater) Picnic they used to make - might be worth a look? Same principle as the Honda Stream I think, and don't Mazda do one as well?
The best thing for me about all the biggies is the height - when we reluctantly got rid of the Space Cruiser (it was 12 years old and costing a lot) I got a Suzuki Wagon R instead because it's still high up - easier to get in and out of, good view out and it goes "coo-eee!" at me in enormous car parks.