Gun emporia in the four border states are the major suppliers of guns to the Mexican gangs. There are more than 6600 dealers, too many to police properly.
See NY Times editorial, "The Drug Cartels' Right to Bear Arms," Feb 27, 2009.
http://nytimes.com/2009/02/27/opinion/27fri3.html?_r=1&th&emc=th

Art. 27 of the Mexican Constitution sets out the regulations on land ownership and leasing to foreigners. One provision is that the Mexican States can vary the rules; see post by Amos, above.
Leases originally were for 30 years, now extended to 50, and renewable. As Amos says financial stability is necessary, and a trust established with a Mexican financial institution.
In addition to sites mentioned by Amos, the old silver towns are popular with foreigners because of the fine climate at a few thousand feet in altitude. As well as Americans, many UK and EU citizens have settled in Mexico.

Years ago, Vera Cruz was very popular with tourists before the west coast got the publicity and the big tourist hotels. A fine old city, and cheaper now than the west coast for expatriates.