Minimum professional requirements

You will need to meet the following requirements to become an IELTS examiner:

 An undergraduate degree or a qualification which can be demonstrated to be equivalent to an undergraduate degree.
 A recognized qualification in Teaching English to Speakers of Other Languages (TESOL, CELTA, etc).
 A minimum of three years full time relevant TESOL teaching experience to adults.
 The required professional attributes and interpersonal skills

They can make a  lot of money but the British Council, I believe, does not provide the visa nor does it provide visa support.  Additionally, a great deal of travel on weekends, usually, can be expected, as many of the tests/duties/etc., are scheduled on weekends.

For someone with a visa-in-pocket and a back-up source of income, it is indeed a good way to line one's pockets but the job can be exceptionally tedious and time-consuming.

None. Some of the companies will give you some noise about requirements but I personally know 3 foreigners in GZ who got IELTS jobs fresh off the plane. Their only qualifications were being native speakers. They didn't even have college degrees yo.