Changing a 3D model runtime

I am making a non 3D game but I want to use a 3D model of a human, can be either male or female. But I also want the user to be able to change their apperance, changing hair style, color, eye color, chest size, bodymass, tattoos and other things. So here my problems lies; The hair is''nt too much of a problem, only thing it to attach it to the head (which can be of different sizes and shapes, anyone know?) Second it''s the general size of things; arms, chest, legs can be either fat or thin. Third is the tattoos, this is the only think im rather sure of, all I need to do is manipulate the texture or the character, i think. Can anyone please give my some clues as to where I can start? Thanks in advance -Thomas Leggett