In response to your inquiry, I add the following suggestions:
1) First, there should be adequate ventilation in the bathroom. You should first check to see if the bathroom has a window. If not, make sure there is a fan to draw the moisture out of the room. This preparation will alleviate many headaches down the road. Being prepared is a must when you are doing a project like this. You don't want to spend countless hours painting a mural and then find it ruined by moisture when that could have been prevented from the beginning.
2) As for hiring a wallpaper crew, you need to do some research. Call around, speak with people who might know or refer you to a business who specializes in hanging wallpaper. Don't choose a company just because it is cheap. You should hire people with the right know-how and expertise. Again, preparing things in the beginning will save you in the long run.
3) As for painting the mural in oils, I would not recommend that. Oil paints are composed of different elements, as compared to regular acrylics. If you want something that is durable, I would suggest that you try acrylics instead. (Hint: Check for mediums that can be added to your acrylic paints which retard the drying process. This will give you more time to blend colors.)
I hope my suggestions have been helpful to you and other readers.
Muralist- D. Windseth