You could use white watercolor and repeat the flashing to make the stars. Also, acrylic paints are not expensive, so get your parents to buy some for you.
Acrylic is easier to clean, but doesn't leave a smooth surface. Enamel is smoother, and will generally last longer, but is hard to clean up if you want to change...
I don't think it matters whether you fight Bowser before getting all the stars or not. As long as you have 120 stars when you defeat him, you'll get Luigi.