i dont know about buying a car in mexico, we shipped ours to costa rica from europe (hamburg), and then drove it up to canada.
try and pm some mexican locals on the forum, e.g., andres_santacruz , he can probably answer the question.
what i mean with "get a local kid" :
there are many of them waiting at the border to help you with the burocratic procedures.
so no need to pose as their father
, they are just know all the officers, and know in what order to go from one post to the next, what paper to get first etc.
the point i was making is that a lot of foreigners think they are just begging and refuse to hire them, but that's not true, they really can help you (e.g., some other tourists needed almost a full day at the border to nicaragua to get accross, while it took us not even 2 hours with the help of one kid.