As I lived in Mexico, I can tell you the country is really attractive. Nice people, many business opportunities and weather such as those you enjoyed in Spain.
However, in terms of administration and paperwork, this is a nightmare…so be patient and don’t give up when dealing with bureaucrats.
In terms of job opportunities, it is important to settle close to a big city. I don’t advise you to live in Mexico D.F because this is a polluted and inconvenient urban area.
In my point of view, Queretaro, Puebla and Veracruz are medium size cities where you can enjoy moderate cost of living and good quality of life.
As your job requires broadband and your husband will need a job, better to be close to a city.
Regarding your budget, £600 a month is not enough as living with this alone in 2004 was just ok. (Check supermarkets prices with Comercial Mexicana.)
Try to visit different Mexican cities first, and avoid those tourist areas such as Acapulco or Cancun.