Thanks. I went back and forth on several different options for the inside. I wanted to do cedar but that was to expensive and the reviews online for what I was going to buy said most of the planks were warped and couldn't be used. I looked at paneling and didn't find anything I liked. I decided to go with birch plywood until I went to a different lumber store. I found 1/4" underlayment that looked very similar to the birch plywood but was less than half the cost. I'm going to stain it after I get it all installed. I think the wood grain will show more with the stain and sealer on it.
Pappy, you are right. I doubt this will ever be 100% finished.