There are tons of papers available that would go with your midcentury home. Many people tire of a paper quicker than they are ready to redecorate and they can also look dated in a couple of years. Metallic papers were big im midcentury...perhaps a metallic silver paper? Are you also considering paint? You can get it in so many textures and a blue wall to match the blue in your loveseat would be lovely with some midcentury art and sculpture, or photographs on it. A mirrored wall with or without art or sculpture would also work with midcentury. These treatments are easier to adapt should you wish to change the look slightly from time to time. I would suggest replacing your stairrail with chrome or acrylic if that is in the budget...see how well the loveseat frame looks with the fabric? I hope you will post the "after" photos for us to see. Above all, enjoy the process!

based on the drawing, the wall where the bed is. HOWEVER, you mentioned that this drawing is not a true depiction of the furniture you will use and the wallpaper. if that is the case, i think the wall paper can work on either wall, it all depends on how busy this wallpaper is and how "plain" the bed is. if you go with a busy wallpaper behind the bed, you may want to keep your bed and bedding on the minimal side so that there is a good visual balance to the overall composition. if you go with snazzier bedroom furniture, then a busy wallpaper may be a bit much and you may want to put it on the other wall or use a textural option to serve as the bed's background rather then another focal point.

Yes, I had noticed a lot of pics on Houzz, that's what gave me the idea! I certainly wouldn't put it IN the shower! We have a tiled shower, but the tiles only go as high as the glass surrounds, so there is about 30cm of painted wall above the shower, which is the part I was worried about with steam etc.

I'm sure there's a lot of information out there thanks to a quick Google.