1.) Depends on what clothing would be most comfortable to wear. You don't want to be wearing anything tight or that will rub a lot.
2.) I just got a watercolor tattoo about a month ago. It's like any other color tattoo; watercolor is just a style. Color in general can hurt more depending on how much the artist has to go over it. Mine didn't really hurt. I'm not sure exactly what you're asking about for experience, so feel free to send me a
PM if you have other questions.
3.) Forearm is probably good, but go with what makes you comfortable. Forearm isn't supposed to hurt too much either.
4.) I would recommend designing it yourself or with the help of your artist. Spend time researching artists who are best at the style and type of designs you want.