I have never bought new here but I have friends that have. It took them 2 days to complete the process because they were foreigners. It mostly has to do with the delay for transferring money from the States to Mexico. There is a huge push for transparency in large transactions and I am sure that is part of the delay. Your best choices for a fast deal will be in Hermosillo which is a great place to hang out for a few days seeing everything there while waiting for the deal to complete, Mexicali is closer but selection will be less, Caborca tends to favor the truck side of the market as it is a agri community. I would plan 3 days for this to take place. Once the car is purchased you have to register it locally. That requires you have a Mexican Drivers License and a local address, you may also need your passport, it takes about an hour to do the registration and cost vary based on the vehicle.