The companies with the best paint products are Benjamin Moore, Dunn Edwards, Sherwin Williams, and Vista Paint. As a painter I don't recommend Berh from Home Depot as much because it's quality is just marketing. Flat paint has no sheen and it shows its true color, but is not easy to wash or clean. Satin, eggshell have low sheen and is washable. Semigloss ang glossy has a high sheen and these paints are recommended for bathrooms and kitchen or where there's high humidity and steam.
If less than half the old paint is left, however, it may be worth stripping it all off. Guertin gets rid of stubborn remnants using shrouded grinders (like the PaintShaver), infrared paint strippers (such as the Speedheater), or chemical strippers (like Multi-Strip), then smooths the wood with a course or two of sanding. When siding (or bank accounts) can't take the shock of a total strip job, Rich O'Neil, of Masterwork Painting in Bedford, Massachusetts, has successfully hidden rough, well-adhered paint under Peel Bond, a thick primer.
Painting contractors work in the property maintenance sector. Depending on their skills and resources, they might specialize in painting different types of buildings, including houses and apartments, or commercial properties, such as offices, factories or retail outlets. Some contractors work directly for property owners; others find work through real estate firms, rental agents and property maintenance companies.
Oh, where to begin? Let me start with 'watered down paint'. 25-50% before the material gets to the site? Impossible. You would basically be painting with water at that point. It would be less of a hassle, and cost, to simply use proper material. You would be forced to apply three coats instead of two, as the coverage would be horrible. Whatever cost you think might be saved in materials would be lost in labor.