No need for plastering! I’ve been a decorator for years and unless the plaster is literally falling off the wall I try to avoid it.
Take off all paper, lining as well. Clean walls with sugar soap and sand down. Then apply a mist coat (20% water added to contract matt), this will highlight all imperfections and allow the top coats of paint to adhere to the plaster. Once all imperfections are highlighted, you can fill, caulk, sand etc till they’re ready for painting. Dependant in how much filling there is, there are different fillers for the job.