I will be repainting bedroom (drywall). One wall was painted black, and and I want to paint it white. What preparations are needed before painting the wall white? Thank you, FAM

The exterior paint on my walls is old and in extremely poor condition. For prep I have seen the suggestion: powerwash>>>scrape>>>prime bare wood>>>paint. After scraping there will be abundant dust and loose edges, should I powerwash again? As the existing paint is in very poor condition (in many spots there is already significant peeling down to bare wood), should I prime the entire house and not just bare wood spots? Would this likely improve my final results? If I do this am I likely to have a better chance at good one coat coverage? Or should I just prime bare spots and plan on two coats? By the way, the old paint color is white and we will be painting with a pale yellow. One last thing. Seems like the opinion here is that Behr paint is not so good and Ben Moore and Sherwin Williams are recommended. Are these second two paints likely to give me a better shot at good one coat coverage with my color choices? What is the difference in price between these “premium brands” and Behr (which is a little less than $20 a gallon). Thanks for any help.