Painting - Is priming necessary? Which paint to choose?

Hello, we bought an old terraced house and have now wetted, filled, and finely plastered all walls and ceilings with MK1. My question is now regarding painting!! Do I need to use a primer, or is it enough to dilute white paint with water and use that as a base layer and then normally apply white paint as the second coat!? How have you done it or do you have experience with this?

Take a cheap primer and spray it on the wall with a plant sprayer. It’s faster, and 5 liters for 10,- Euro is nothing.

Always apply primer first on plaster.