Changing from Two Wheel Drive to Four Wheel Drive

Has anyone got any idea how much it would cost to have a two wheel drive Bongo converted to four wheel drive?

We have been looking for the 'right' Bongo since October/November last year without success, and now that we've found what we hope to be 'the one' it turns out to be two wheel drive and we want four !! We don't want to dismiss the idea of this car totally, so we are considering all options.

Thanks.