How well does the design of prototype have to be arranged?
A so-called function design in principle suffices, so you do not have to engage a professional graphic artist or illustrator to do that. It depends on the idea not on your design in the end. If the game should be realised, then the publisher has his own ideas certainly about the topic, the design, the formats etc. You may nevertheless take a little trouble with the prototype – test players and editors are also people with whom the eye joins in. It is really important, though, that all functions are recognisable openly and clearly for the game.