I just bought a horse in Mexico last week. I went there in March and looked, specifically in Guadalajara. I answered the OPs specific questions by PM.
My experience was great. I was escorted throughout my visit and never felt unsafe. I worked directly with the breeder/owner whose ranch I was visiting. We flew out of Juarez, MX as it is much less expensive than flying out of a US location. We live just outside of El Paso, TX, so it was easy. Another hint is to fly Southwest or another US carrier to one of the coastal cities and then catch a domestic flight to the interior of Mexico. Puerto Vallarta is the closest resort area to Guadalara. Flights from there run around $50 last time I checked.
Regarding transport/quarantine: The quarantine for horses from Mexico is 7 days. There are no differences for geldings, mares, or stallions. All seven days. The transporter I will be using is based in Riverside, CA and he will deliver directly to my door, he is who the seller uses all of the time. There is another transporter who owns his own quarantine station that is based at the border crossing across from Santa Teresa, NM. He has a transport company that goes to Mexico, US, and Canada. I’m sure he could arrange for transport from Santa Teresa to other locations in the US. PM me for names and contact information.