Wood in the bathroom?

Howdy folks. It's time to start planning my spring project. And of course no project is complete without problems. I have decided on the upstairs bath. The previous owners thought they were do-it-your-selfers. And oh boy did they do it! Everything they touched was as shoddily and cheaply done as possible. Until I finish the downstairs I can't change the layout of this bathroom due to where they ran the new plumbing. I absolutely despise that hump thing behind the bathtub. But that is where they put the new stairs and for now I am stuck with it. I want to build a shelving unit that covers it up and rises up behind the tub and a little up the sides of the tub. I want the first shelf to curve around the back of the tub and fit up against it and rise up in kind of like a tri-fold around that back. What kind of wood would you recommend for this kind of job? I am going to paint it when it is done, I was thinking with exterior paint. What is more important type of paint or type of wood? This bath has no shower and gets minor use. Furthermore in a few years it will be gutted and restored correctly. I just need a little cosmetic fix so I can stand to use it while I replace the downstairs closet...I mean bath.

Here is a link with some photos so you can see what I am talking about. And while you are at it what is up with that pipe in the corner. It is cut off from below but still comes out the roof on the top. What is it? Sewer pipe? Can I get rid of it?