You can create any character you like without buying anything, unless you want hair other than what you have right now. The hair is the only thing on a custom character that requires you or the user to "own" that hair that you want to use. So for instance if you want to use the feathered hair that Ulani has, you need to own Ulani, and anyone wanting to use your character in the way you intended does as well. Everyone has "glamour" hair and "pixie" hair for females, so if you don't have any other hair then your characters will have to use one of those (or "none" for bald).
You can also make any clothing you like. So besides the hair, you can make as many characters and outfits as you want without having to purchase anything. Just follow the tutorials here in this section of the forum and that's all you need.