Frank, you might find it can be almost prohibitively expensive to retrofit the rear seat to an existing cart.....if you go the further steps to have the extended roof installed (requires a complete replacement of the original one), the Sunbrella package completely redone, and heavier duty rear shocks installed. We considered that as a potential option when we were tossing around golf cart ideas last year and ended up dismissing it purely over costs.
Best bet money-wise would be to find one that's already set up that way.
Bill