If you do a trawl through the past pages of this forum you will find much discussion of the questions you pose.
A short summary would be:
1) There is always a demand for FIs, but not enough to make it a seller's market;
2) Pay is very low (<£500 per month) for long hours of sitting around waiting for the weather to improve;
3) Most people advise doing some job that gives you a steady income and then fit in the instructing around that. Trying to make a living (and pay off a loan) from instructing alone is very difficult;
4) Expect no loyalty or reward from most schools; they see you as someone who is lucky to be allowed to use their assets on the way to an airline job;
5) Several schools do FI courses, I can't recommend any. Depends really on where you live and how much you are prepared to travel.
Good luck!
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"Take-off is optional, Landing is mandatory"