Apologies if you know all this already, but many people don't consider the implications fully.
In an accident, there's almost no space between the rear-most passengers and whatever hits the back of your car. The passenger safety is further compromised by the fact that they are sitting in the middle of a space that is designed to be a crumple zone.
This is not specific to the Berlingo, and it applies to lots of other so-called "seven seaters" as well. The public's safety yardstick, Euro NCAP, doesn't test for rear impacts, so the implications are somewhat hidden. Personally I think that if a manufacturer offers a seven seat option, it should be specifically crash-tested by Euro NCAP so that buyers can easily gauge the risks and make meaningful comparisons between car manufacturers and models. Euro NCAP has been hugely successful in modifying manufacturers' attention to safety, but arguably, more so only where it is tested and published.
To answer your specific question though; if you want to do it, then probably the best way would be to use original factory parts, as per the "Family Pack" option, and buy a roof box for the luggage. You wont be able to leave the overhead locker in the boot (this is removed in factory 7 seaters).