Well I'm no expert but I'll try and offer some guidelines ... pretty much how we went about locating our van.
Onroad/ Offroad ... Where do you want to go with it? If it's mainly black top work then there's not much point spending the extra $$$ on an offroad van. Even on a gravel road most of the 'onroad' vans will handle it ok so long as you take it easy (which you have to even in an offroad van). We have an 18ft Jayco Heritage and it handles most gravel roads well. I'd have no probs taking it on the Ayres Rock/ Wiluna road but maybe not Poepple's corner due to the sand dunes.
We recently went to a beach camp spot here in QLD (we had our CT) and becuase of the overhanging trees a friends Rapid sustained a bit of rooftop damage. I've seen some vans on Rainbow beach and they inevitably become bogged and to be honest I'd rather recover a 4wd instead of a van.
Size ... do you have a family or is it just for you and your wife/ girlfriend/ mistress? Are you going to do long trips (several months or more) or just short trips (up to a month)? For us, we went with an 18ft rig simply because it had tandem wheels which sits better on the road and is much more stable if you have a blow out. Also, what is the tow rating of your car? even if it's say 2500kg's (like mine) then you really don't want to get anything over 2000kg TARE as you will add heaps of stuff for your trip ... eg. food, water, clothing, chairs, tables etc. I allow 200kg per person so for us with a van of TARE weight at 1700kg's that gives us a gross weight of 2100kg's. So it's well within the vehicle's limits. The other issue with size is where do you want to keep it? Will it fit in your yard or will you have to store it elsewhere?
Pop top vs Standard van. Not sure where you live but we find the pop top a must. Here in QLD it can get very muggy so with a pop top the interior is much more 'breezy'. A pop top is just as strong as a normal van (the sides are reinforced) but generally a pop top is the same price as a standard van. Also, if you want a toilet / shower set up then a standard van maybe the way to go, but you can get small cubicle shower/toilet set ups in pop tops. SO far as fuel economy is cocnerned I've heard alot of ppl say a lot of thngs. I've only had the pop top but prior to that I had a Jayco wind-up (Eagle outback) and to be honest I don't notice a great deal of difference in fuel economy when towing. I'm trying to find out more info by finding someone who has had a poptop and then bought a normal van but so far no luck.
The other thing is interior. We didn't want the standard 'L' shaped lounge which made things slightly harder but what we ended up with is great for us. It's a large dinette lounge that converts into a bed and as we just have the one son with us it's great for 3 ppl but if you are a family of four then you might want a van with bunk beds. Ensuite is up to you but it does add $10k to a new van price ... ensuite sites at van parks vary in price but I worked out it would take 3 solid years of touring to recoup your money. Even a 2nd hand ensuite van is quite expensive.
In regard to the Expanda what I dont like about them is the canvas roof over the bed. If you're packing up and it rains your mattress will get wet. That's another thng ... it's only a foam mattress ... standard vans have innrespring mattresses. The Windsor Rapid also has foam mattress ... something to think about if you intend doing long trips.
I'd better leave it there ... hopefully this post will generate some discussion and aid you in your choice. I'd also look at caravan mags ... Caravan and Motorhome plus Caravan World are probably the main two.
Oh ... don't forget accessories such as electric brake controller (Prodigy is approx. $300 fitted), weight distribution hitch ($600 new, approx. $200-$300 2nd hand). Mirrors are good too ... approx. $150 - $400 depending on what you buy.
HTH
Good luck!
Scol.