My Cousin wants a Vocaloid, which one should I buy?

I suggest searching through YouTube with your cousin and listening to different Vocaloid songs. Here is a list of Vocaloids I've compiled that you can look through;

IA, IA alpha type C, IA Rocks, miku, miku dark append, miku English, maika, akikoloid, anon + kanon, rin + len, aoki lapis, Gakupo, gakupo append, Clara + Bruno, daina, Dex, Flower(V4 or V3), Galaco (red, blue, NEO), Gumi (native,adult,sweet,native,power,whisper,english), kokone, Lily, luo Tianyi, Macne nana (eng + jap), mayu, luka, meiko, merli, mew, iroha, otori kohaku, rana, ring suzune, sachiko, seeu, sf-A2 miki, sonika, tohaku zunko, tone rion, uni, xin hua, yanhe, yohioloid, yuezheng longyou, zhanyin lorra, yuzuki Tulare (onn, lin, jun) , Zola project, avanna, sweet Ann, Leon, Lola, Cul.

Those are the ones I know of so far. Don't forget to get the VOCALOID program too, or else you'll be sitting ducks.
Not to mention, this will be a few hundred bucks. Be wary of what you buy, but I have to agree with the other answer. Not a lot of people tune Miku, so they've made Miku' s voice sound great on its own. Miki is hard to use.
Also, pay attention to what language the voice bank is. Does your cousin want to make Japanese songs? English? Spanish? Maybe all three?
Pay attention to how many languages these Vocaloids can sing in.

Have fun!!