Paint is about the only subject I know as much as Paul, so I thought I'd weigh in.
Artist's acrylics have a different binder than hobby color, and it doesn't bond as strongly to plastic. And the more water you add, the weaker becomes the paint film; so it's advisable to add the minimum of water required to get the flow you want.
As for the brand, the price of the paint is dictated by the pigment: both quality and quantity. Cheap acrylic, often marketed as "student" paint, has more of the acrylic binder which makes them have a glossier film. And they substitute the more expensive pigments with synthetic imitations they label as "hue". Example: Cadmium Red Hue has no cadmium, it is made of a cheaper substitute.
WINSOR & NEWTON makes a very good paint, but it's expensive in the States. GOLDEN has about the same quality, and being made in the U.S.A is cheaper. LIQUITEX is almost as good, and cheaper still. I use many brands, but LIQUITEX most often. (The best quality acrylic paint in the world is made by the Swiss company LASCAUX, and it's VERY
expensive.)