A double cab pickup may seem like a good idea (and they do look butch and flashy!), but in reality, you will struggle to find a 2wd one, and unless you need 4wd, that's going to be much less efficient than a standard transit size van. Also, pickups are big cumbersome machines, which could be a problem in tighter streets. Do you really need 500l?, because if you think you could manage with 350l, a lwb Renault kangoo crewcab might be worth a look, and with a 1.5 turbo diesel, it will be pretty thrifty. They have a payload of about 750kg though, and I think you would be pushing the limit to put a 500l tank in there. I have a 500l tank, but I am pretty certain I could get away with 350, so when the time comes to replace my van, I will certainly be bearing that in mind. If you really need the 500l, any crew cab van with a payload of at least 850kg (900+ would be better, just to be on the safe side) will fit the bill.