I've had lots of vans over the last 8 years, transporters, vitos, trafics, vivaro, sprinter, iveco, lt35 etc and have seen a load more so I've got a pretty solid background to comment on these things.
If you are set on a transit/transporter sized van then you'll be very lucky to get 40mpg consistently. New shape trafic with the 115 engine should be touching that at times and some of the t5 transporters will do it to and a bit more if driven like a proper grandad.
At your budget however forget transporters and vitos. Vitos will be in price range and may look tempting but are pretty horrible places to be (shortest van i ever owned was one, 2.5 months to sell it on as it just wasn't nice to be in), suffer really badly with rust and aren't very well built. Transporters just have too much scene tax to be a possible at that price range, a shame as they are the nicest van to use - just pipping the vivaro/primastar.
I'd be looking at either vivaro/primastar/trafic (all made in the same factory in luton, same bits, engines etc and just different badges) or a transit depending on what you can find for your money. Both vans have probelms, the weak points on vivaros are the gearboxes, they are better on the newer 2.0 engines than the 1.9 but still aren't the best. If you find a highish mileage one 100k ish then if it's not already had one budget for one at some point, mine cost £750 fitted for a recon. Transits seem to suffer more with injector and fuel pump problems costs similar. For me the trafic group are nicer vans overall and are good to drive. Look for vivaro sportive if you want a nice looking one, plenty around and the premium often isn't too high. For a normal one a 08 or 09 should be within budget without silly miles.
If you have any specific questions drop me a line and i'll try and help out.