Decks, Patios, Porches and Docks - 3 Season room question

The house my wife and I bought has a 3 season room that was built in 1961, so it's about time it gets touched up a little. On the wall connected to the house, i removed the plywood paneling to find the old shingles. I am going to remove the shingles also, but want to know the correct size of sheetrock i must put on that wall. Also, should i put plywood up before the sheetrock? The exterior wall is made up of boards about 1 inch thick, and 9 inches wide spaced about 1/4 inch apart from each other. The entire wall is about 2 inches deeper than the door frame, so i have plenty of depth to work with. I want to finish the wall with wainscoating on the bottom and painted sheetrock above. I am not going to insulate this wall because except for the door into the house, this wall is opposite the basement stairs. So, to summarize the questions:

What size sheetrock should i put up, and should i put plywood up before the layer of sheetrock?

Any help would be much appreciated.
Thanks,
Pat