That's plenty of space, and 250W HPS will be perfect. Two plants would be a good number, so grow 3 because what are the chances of all being girls? Bear in mind that many seeds do not even germinate, I recommend getting started right away with 5 seeds in a cup of water.
A great tip for closet growing is get yourself some of that foam strip for keeping the doors air-tight. Will you need to make a carbon filter, or does the closet intend to be a skunk-scented aerosole for your humble home?