You should focus on getting the right model S, cost of rear seat is as following:
Seat from eBay or some seller here: ~$1000, all the parts including the bumper cross member directly from Tesla service center ~$425, installation of the parts by Tesla service center ~$900 or third party installation $400-$600. So you are looking at about $2000 ballpark cost for adding rear facing seats.
They are amazing, kids love it.