The center console and dash etc is a straight swap over, 30 minute job if you know what you are doing. As for the rear seat ... the base is no problem, fits perfectly, but the back is a different story. I had to get it re-trimmed in my car. It is about 5cm short on each side because the Statesmans have the additional panel which is an extension of the door trim (being LWB). There is also two holes on each top corner for the seat belts as they come through the seats on the stato, not from the C pillar. If you know an upholsterer they can do it for you cheap (saving on labour) otherwise some guys charge a fair bit for the extra foam and "effort" required. As accentstencil said, have a look at the link to my ride in my signature below and you can see the photo's of it in my car.
My suggestion (if you can get matching leather) is to buy a back from a VX Calais and base from WH Statesman!! Either way it will require a bit of effort, but nothing is impossible!