By posting a painting question you will now hear from half the world. As far as wall prep, the easiest thing to do is to use a liquid deglosser. I would not use the oil base paint unless you have a long time to wait for it to dry. The latex paints are effective in bathrooms and will dry much faster. Since I'm not a pro painter, but do a lot of painting, I go to a professional paint store. You know, like Benjamin Moore, Sherwin Williams. They will take the time to guide you to the most appropriate products and offer preparation advice. I use the big box stores for a lot of things, but paint is not one of them. Not that they are lack competence or quality, they just simply can't or won't take the time in my experience.
I'm sure there will be a number of other opinions posted. Good luck with the project.