The question is ... is it paint or coloured render?
If render, then a scrubbing brush and bleach. Only use a pressure washer if it's fairly recent sand & cement render.
If it's an older house with a lime render, then no pressure washer.
Anything is paintable with the right preparation. Firstly, try a small area with a good scrub. See how it comes up.
From your description, it sounds like a gable end. I can't answer the scaffold question without being there. If an experienced decorator thinks he can handle it with towers/ladders, then he'll do so. Scaffolding isn't always necessary.