The choice of van is yours, dependant on yor budget, but there's some excellent advice here.
One thing to use as a bargaining tool is the awning. Make the dealer throw one in as part of the deal, otherwise walk away. An awning can be the difference between as much space as possible, and a fractious holiday, especially in inclement weather. Kids love awnings, and it doubles the size of your living area.
We now own a 2002 Elddis Avante twin wheelbae model, and find it to be everything that we have ever wanted, after many years of older and smaller vans. There are usually some good deals on the twin axle models just prior to the Caravan show, as dealers reduce their stocks. We picked ours up quite recently, and we've been very lucky, as the previous owners have clearly been exceptionally careful owners, and the van is literally in 'as new' condition.
I personally agree with the comment regarding your Espace. If you are set on a twin wheelbase model, then in my opinion it won't have the guts to pull it except on anything but a very flat road. We pull ours with a Ford Scorpio Ultima 2.9 Litre.
You need to carefully research your weight considerations re vehicle/caravan, before taking the plunge.
A good idea would be to visit the annual Caravan and Motorhome Exhibition at the NEC 16 - 21 Oct 2007. Here you would be able to view everything under one roof, ask all your questions, and even tie up a deal.
Finally, I certainly wouldn't discount a recent second hand van.
OK, it's nice to have brand new, but at the end of the day, you could save yourselves a LOT of money like we did, and still have a superb caravan. Caravanners are a pretty down to earth bunch, and I'm just as happy to chat with the owner of an old van as with the owner of a brand new one. Doesn't bother me what they own. We're all caravanners at heart.
Just take your time to weigh up everything before you part with the cash. With a Dealer buy, you'll get the guarantee etc. With a private buy, you may well get several hundred pounds worth of extras thrown in. It's all a personal choice.