Apoolgies for the thread revival, but just seen it.
Its not that hard a conversion for an upholsterer. Give me your email address (and a few days to dig the bits out) and I'll send you some pictures of whats needed. If you can, get a rear centre section from a scrap yard as one will need to be cut (for rear vnts) to give a decent/profeesional finish. If you cut the one already in your car, when it comes to selling it, you wont have the option of putting the car back to standard. With regard to the seatbelt, you can get either a new one or one from a scrapyard.
The main reason for doing mine was also the need for a 5 seater for family. I know there are alot of people that talk about safety issues, but I dont see it as a problem. I cant see the addition of probably less than a 100kg (5th person in car) making that much of a difference as long as the seatbelt is fitted properly.