As I understand it, all still needs to be processed by a Notario, except that formal processing will take place after the selling/ financing agreement is satisfied. If the property is held in a Mexican Title you first need to be sure that it can be legally sold. If title was granted to a family by the government there can be restrictions. Also you need to be sure the person selling it is one and the same as the person named on the title. Get a financing agreement signed and agreed to by both parties. You could have a Notario draw it up or use one of the standard forms found on the net. I am pretty sure it can be recorded at the City but many avoid this as it causes some issues, expenses and delays at the time of transfer. If it is held in trust by the seller at the time of sale, the trust will remain in place until the note is paid, the bank will not even be aware of the sale. The annual trust fee needs to be paid and the utilities ( and liability to pay) will remain in the name of the trust holder. When the note is paid off there will be a charge by the bank to cancel the trust. Who pays that should be specified in the financing agreement. When the note is satisfied the Notorio takes over the process as similar above, except he processes to a Mexican title not a bank trust and it should be less money in closing fees. Be aware that there apparently have been some changes made to real estate transactions and the responsibilities of the Notario in this process. Talk has been that the money will have to pass through a Mexican bank and not directly from buyer to seller as it has been. This is designed to interfere with money laundering. How that might effect a future 'slosing' is indeterminate until this process is described.
I am neither abogado or a licensed realtor. The above description is based on 7 years or so of observing a variety of real estate transactions and may not be completely accurate but should indicate the issues that need inquiry and attention. You have to go with the Notario, no one else can make a transfer. Know also that AMPI has nothing whatsoever to do with the legalities of real estate transactions in Mexico and membershiop does not guarantee anything. I will share some stories but only privately with you buying !!