All depends what kind of layout you are looking for, but I would recommend you take a look at an Adria.
We have the Adria Altea 542 DT and it's brilliant for us and the kids. Large triple bunks and a large shower / washroom means it's perfect for us and I wouldn't change it for anything else at the moment.
Also the Adria Altea 542 DT is worth looking at.
They are great vans (Ask any Adria owner) and are very light and stable to tow.
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go for a second hand Adora 642 up bit heavier large well built giant fridge freezer two single fixed beds or make double ,two singles for kids at front or make double privacy screen but no probs for xtrail vw tiguan. if serviced regular still part ten year water engress warranty.
your choice come down to something as big as you can possibly tow or as small as you can get away with.. picking something in the middle of the two extremes is probably the best idea.. ;)
In terms of the X-Trail I would advise you to go to a weighbridge and see what yours weighs. We found it hard to get a firm figure on the actual kerbweight of the car on line as figures varied a lot. Ours came in at 1820kg with 3/4 take fuel and me in the drivers seat and that was with an empty boot. I now work to 85% of this figure which pretty much means most single axle's are fine. In reality it will also be more than this when towing as we would have luggage, etc plus the family.
In relation to your choice of caravan I would have a look at a Bailey Ancona. This is the layout we are looking at next as it has the twin bunk beds (triple have started to drop out of favour with some due to the limited head height as the kids get bigger) and then has the option to convert the side dinette to another single bed with bunk above (its a 6 berth). We also like Adria but like to have a seperate shower which is way the bailey appeals. If you get one with a full service history you will be able to transfer the remainder of the warranty. For your budget you should be able to get one around a 2011 model and as these came with a 10yr ingress warranty you will still have 6-7years left!