Waldo,
I do not know that there is a separate kit that you can buy to install a third seat. Back in the day it all came in one big box and if you needed items individually you just ordered them that way. I suggest you find the best looking third seat that will fit your application and remove it with all of the necessary stuff to put it into your car. On the 7/900 cars it is about as simple as it gets. Bolt in the seat, pull two trim panels and grind one to fit, install one piece of trim where you ground away the necessary plastic to make it fit, install your seat belt reels under the panels you removed, run the anchors up to the roof where they bolt down under the little panels that just pop out, grind those little panels to clear the seat belt anchor, run the other end with the anchor on it down to the top of the rear wheel well and shoot them into the existing well nuts (on the left one you will see where the grinding away now facilitates the lower seat belt anchor). Now your seat is installed, panels cut to fit, belts installed, reinstall your side panels and you should be pretty much done.
It has been probably 7 years or so since I have done one, but I did a boat load of them at the time, so I am sure I glossed over a point or two but that is the basic gist of the operation.
If it were me I would lean more towards a 7/900 series car just because they are a later model and will generally be in better shape and have a little bit better parts availability, not much though.
Where to get the reinforcing kit? You know that is a good question. They are made by one guy out of Canada (I think?). Look around at your local online parts sources and try IPD. I get mine from a company called Ramac but they do not deal with regular customers, you have to be a shop owner/in the business kind of guy. Last time I checked they were in the area of $20.00.

Mark