First off, the picture on the top THAT IS A HOME OWNERS PAINT JOB. If you here a school kid or your neighbor, this is what you get. I was a painting contractor for the better part of 40 years and never saw a PAINTER (even the worst painter) leave a mess like that. Maybe the electrician or the carpenter but, that is not something a painter could even do if they tried.
If you're going to do any part of the painting or prep work on your own, you must know that the first step in preparing a surface is washing it. Since dirt can affect the smoothness of a surface area -- and therefore paint's adhesion to it -- use soap to remove any dirt or stains. Make sure there's no soap left on the walls when you're finished; also make sure to remove any gloss with sandpaper and vacuum up the leftover dust.
Consider purchasing supplies personally to save money. Ask the painter for a bid that separates labor and materials. Then explain that you'll purchase the materials and ask for a list of exactly what will be needed to complete the job. Caulking, for example, is an extra supply commonly used to fill any cracks or damaged areas in your walls -- and one that might be overlooked in an incomplete list.