I'm lucky to have bright coloured hair so I just pre-plan to dye my hair pink or green or whatever I need it to be before cosplaying,
as for the outfit I'm normally pretty lazy and buy a dress or a skirt I like and sew the cutie marks/other bits I want on it by hand. I just get the correct colour in clothing.
I'm in the process of making my own wings for a cosplay out of wire and several hundred feathers!
Good luck with it, ebay's always a good place to start, pick coloured extensions for a tail and you can go from there!